ADD: echo classification

Adding echo classification module, including a Fortran routine for the
method of Steiner et al. (1995). The echo classification module should
be general and host any number of methods (e.g. Steiner method,
Biggerstaff method, etc). This commit does not include the necessary
object and/or pyf files required to actually run the code, as I figured
@jjhelmus would be best at handling this. I assume some modification to
the setup.py file in pyart.retrieve is in order...
MOD: minor changes
ADD: echo class wrapper

Adding f2py wrapper to the Fortran Steiner echo classification routine.
According to the SciPy docs, the distutils extensions in NumPy can be
used in setup.py to define an extension module entirely in terms of this
pyf interface file, and is the best way to distribute f2py extension
modules.
MOD: minor changes

Adding some minor changes, as well as some more documentation. The
documentation is still unfinished though...
DOC: updating documentation

Updating the documentation for the Steiner echo classification routine,
including the appropriate reference. I also checked echo_class.py
against PEP8 standards, and it looks good, except for several "blank
line contains whitespace" remarks.

One thing left to do is add the appropriate extension to the setup.py
file...
Merge branch 'master' into wip_pr_135
MAINT: Refactor of Steiner echo classifier.

* Created a setup.py file which conditionally compiles the Fortran 90 module.
* A pyart.retrieve namespace was added to which the steiner_conv_strat
  function was added.
* Function added to the documentation.
* Unit test added, new grid object added to testing module.
* Bug with masked array fixed  in steiner_conv_strat function.
* Extension module renamed to _echo_steiner.
MAINT: Changed parameter in steiner_conv_strat.

* dx and dy now default to None, which calculated resolution from
  grid.
* reflectivity now used as default field.
Merge pull request #3 from jjhelmus/pr_135_enh

Including Jonathan's refactoring and minor bug fix. See his commits for more information.
Merge pull request #135 from kirknorth/echo_class

ADD: echo classification