add Feature.astuple()
new iterators module for more generic db creation

(lays the ground work for db.update())
change feedback when verbose=True to better support iterators
update create module for iterators that always yield Feature obj
send dialect when creating Feature from line
order_by kwarg for FeatureDB.features_of_type()
store observed dialects
add iterators.StringIterator
create.py: s/parser/iterator/g
add support for StringIterator in create_db
default attributes should be DefaultListOrderedDict instance
support for creating Features from a line even if no attributes
now use iterators.py instead of parser.py for iterating through data
add helpers.dialect_compare
add helpers.make_query for more uniform kwarg handling and query creation
refactor FeatureDB to call helpers.make_query;

store _last_query and _last_args for debugging
update doctests and unit tests to reflect recent changes
comments on forcing a dialect check
remove leading "##" from directives
add DataIterator, which returns an Iterator of the correct type