Include property_names parameter in extraction

Allow user to specify which properties they want returned by including the property_names parameter in extraction.
Updated get_params to take property_names
Added test for property_names in extraction
Fixed extraction in __init__
Merge pull request #28 from dantaylor688/patch-1

Include property_names parameter in extraction
update docstrings for property_names
bump to 0.3.1
reuse session for a single API instance and expose it
convert tabs to spaces
bump to 0.3.2
Merge remote-tracking branch 'upstream/master'

Conflicts:
	keen/client.py