Update GeoPandas objects to new pandas API

Define the new `__finalize__()` method (see #36) and `_constructor` attributes.  A few other changes were necessary, and it revealed a bug in the custom `fillna`, which I removed since it isn't necessary in pandas master.

There are now 3 failing tests on pandas 0.12, however.  It looks like they are all related to fillna, so that might have to be reimplemented to keep 0.12 support.  I won't merge until this is fixed, I don't think we need to abandon 0.12 quite yet.

This has the `loc`, `iloc`, `head` and `tail` tests are passing again.