Speedup to_dict with sqlalchemy.inspection

I've spent some time on figuring out what could be done to improve the performance of
flask-restless. Since most of the time consumption seems to come from to_dict I started from there.

It shortly showed up that the way the columns of the current result are being retrieved is not optimal.
The hybrid_method stuff is rather expensive, even if no or just a few hybrid properties
exist.

Since sqlalchemy 0.8 there is an inspect() method (sqlalchemy.inspection.inspect) which does what
has previosuly been done using mro & isinstance and the object_mapper calls.

Execute time has been reduced about 20-50% depending on use-case.

I've create a rather simple test case:

	class Parent(db.Model):
		id = db.Column(db.Integer, primary_key=True)
		name = db.Column(db.String(10), unique=True)
		childs = db.relationship('Child')

		@hybrid_property
		def test(self):
		    return '%s-test' % self.name
	class Child(db.Model):
		id = db.Column(db.Integer, primary_key=True)
		name = db.Column(db.String(10), unique=True)
		parent_id = db.Column(db.ForeignKey('parent.id'))
		parent = db.relationship('Parent')

Create a Parent, add 100 Child's and pass the parent (with deep for childs) to to_dict.

Here is a line-profiler result of the unoptimized version (upstream/master @1d81d6486fef4fe8c5b0051ec772e2f53dc16eb3):

	Function: to_dict at line 228
	Total time: 23.377 s

	Line #      Hits         Time  Per Hit   % Time  Line Contents
	==============================================================
	   [..]
	   270     50500       134320      2.7      0.6      if (exclude is not None or exclude_relations is not None) and \
	   271                                                       (include is not None or include_relations is not None):
	   272                                                   raise ValueError('Cannot specify both include and exclude.')
	   273                                               # create a list of names of columns, including hybrid properties
	   274     50500       118004      2.3      0.5      try:
	   275    252000      1259072      5.0      5.4          columns = [p.key for p in object_mapper(instance).iterate_properties
	   276    201500       613966      3.0      2.6                     if isinstance(p, ColumnProperty)]
	   277                                               except UnmappedInstanceError:
	   278                                                   return instance
	   # first hotspot here:
	   279    252500       702816      2.8      3.0      for parent in type(instance).mro():
	   280   2121000      5514722      2.6     23.6          columns += [key for key, value in parent.__dict__.items()
	   281   1919000      5627984      2.9     24.1                      if isinstance(value, hybrid_property)]
	   [..]
	   298    202000       602245      3.0      2.6      for key, value in result.items():
	   299                                                   # TODO We can get rid of this when issue #33 is resolved.
	   300    151500       520331      3.4      2.2          if isinstance(value, datetime.date):
	   301                                                       result[key] = value.isoformat()
	   302    151500       476480      3.1      2.0          elif isinstance(value, uuid.UUID):
	   303                                                       result[key] = str(value)
	   #  next hotspot here:
	   304    151500      5372080     35.5     23.0          elif is_mapped_class(type(value)):
	   305                                                       result[key] = to_dict(value)
	   [..]

Here the exact same code executed with inspection:
	Function: to_dict at line 230
	Total time: 7.14106 s

	Line #      Hits         Time  Per Hit   % Time  Line Contents
	==============================================================
	   [..]
	   272     50500       123470      2.4      1.7      if (exclude is not None or exclude_relations is not None) and \
	   273                                                       (include is not None or include_relations is not None):
	   274                                                   raise ValueError('Cannot specify both include and exclude.')
	   275                                               # create a list of names of columns, including hybrid properties
	   276     50500       124745      2.5      1.7      instance_type = type(instance)
	   277     50500       113436      2.2      1.6      columns = []
	   278     50500       111525      2.2      1.6      try:
	   279     50500       785644     15.6     11.0          inspected_instance = sqlalchemy_inspect(instance_type)
	   280     50500       323395      6.4      4.5          column_attrs = inspected_instance.column_attrs.keys()
	   281    303000       841419      2.8     11.8          hybrid_columns = [ k for k, descriptor in inspected_instance.all_orm_descriptors.items()\
	   282    252500       646694      2.6      9.1              if descriptor.extension_type == hybrid.HYBRID_PROPERTY ]
	   283     50500       141898      2.8      2.0          columns = column_attrs + hybrid_columns
	   284                                               except NoInspectionAvailable:
	   285                                                   return instance
	   [..]
	   302    202000       519672      2.6      7.3      for key, value in result.items():
	   303                                                   # TODO We can get rid of this when issue #33 is resolved.
	   304    151500       459529      3.0      6.4          if isinstance(value, datetime.date):
	   305                                                       result[key] = value.isoformat()
	   306    151500       433290      2.9      6.1          elif isinstance(value, uuid.UUID):
	   307                                                       result[key] = str(value)
	   308    151500       380193      2.5      5.3          elif key not in column_attrs and is_mapped_class(type(value)):
	   309                                                       result[key] = to_dict(value)
	   310                                               # recursively call _to_dict on each of the `deep` relations

Both of the previously recognized 'hotspots' are no longer that heavy. Due to inspection we can also
re-use some of the information gathered for the column list in order to avoid expensive type-checks.

Even thought all tests are still passing I'm not 100% certain this optimized code works in all
circumstances.

Tests had been performed by using the following script:

	from flask import Flask
	from flask.ext.sqlalchemy import SQLAlchemy
	from flask.ext.restless.helpers import to_dict
	from sqlalchemy.ext.hybrid import hybrid_property
	import line_profiler

	profile = line_profiler.LineProfiler()
	profile.add_function(to_dict)

	app = Flask(__name__)
	app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite://'
	db = SQLAlchemy(app)

	class Parent(db.Model):
	  id = db.Column(db.Integer, primary_key=True)
	  name = db.Column(db.String(10), unique=True)
	  childs = db.relationship('Child')

	  @hybrid_property
	  def test(self):
	    return '%s-test' % self.name

	class Child(db.Model):
	  id = db.Column(db.Integer, primary_key=True)
	  name = db.Column(db.String(10), unique=True)
	  parent_id = db.Column(db.ForeignKey('parent.id'))
	  parent = db.relationship('Parent')

	db.create_all()

	parent = Parent(name='parent-%d' % 1)
	db.session.add(parent)
	db.session.commit()

	for ci in range(0,100):
	  child = Child(parent_id=parent.id, name='child-%d-%d' % (1, ci))
	  db.session.add(child)

	db.session.commit()

	if __name__ == "__main__":

	  from timeit import Timer

	  profile.enable()
	  def test():
	    to_dict(parent, deep={'childs': []})

	  timer = Timer(stmt=test)
	  print timer.repeat(5, number=100)
	  profile.disable()
	  profile.print_stats()
	  profile.dump_stats('to_dict.lprof')