Use setuptools instead of distribute

Distribute is now [deprecated](http://pythonhosted.org/distribute/), and development has moved back upstream to setuptools. Some Linux distributions have stopped shipping compatibility layers for distribute, so the `install_requires=['distribute']` results in fido failing to import distribute at runtime.

Since fido works fine with setuptools, and `pip install distribute` just returns a compatibility shim around setuptools, I think it's safe to just specify setuptools as the dependency now.