Fix "maximum recursion depth exceeded" in Feature.properties
PEP8 on feature.py
Keep id as first kwarg of Feature.__init__ for backward compatibility

Many formats instanciate feature with Feature(id) without using a kwarg.
The correct fix should be to use explicit kwarg every where, but this
has to be covered by unittests, which are mainly missing at the moment.
Merge pull request #7 from yohanboniface/fix_feature_property

Fix "maximum recursion depth exceeded" in Feature.properties (+ PEP8)