support excluding fields
fields={'_id':0} should return full document with '_id' excluded
tests and fix for find(spec, fields) unsupported behavior fields now supports:
{"_id":1, "field":1}
{"_id":1, "field":0}
{"_id":0, "field":0}
{"field":0}
{"field":1}
compare_ignore_order: provide a default sort if there is no "_id" in the document
Python 3 compatibility
cast field.values to list to prevent error:
TypeError: 'dict_values' object does not support indexing

set basestring = (str, bytes) to fix:
NameError: global name 'basestring' is not defined
kicking off my first buid
Merge pull request #26 from texuf/master

more support for collection.find()