Fix issue where `iterkeys` does not decode keys but `keys` does.
Make compatible with Python 3.
Merge pull request #9 from b4hand/fix-iterkeys

Fix issue where `iterkeys` does not decode keys but `keys` does.
Support object types as values.

Fixes #7.
Inlined `self.iterkeys` invocation in `__dealloc__`.

Removed extra `Py_XINCREF` in `_fromvalue`.
Demote part of docstring to comment and remove duplication.
Drop unused variable.
Use `hattrie_iter_val` directly when destroying values.
Fix segmentation fault.