Add tests

[slezica]

@ppolewicz @lurch @eugene-eeo

This project is seeing some activity lately (yay), I think it's time to add some tests.

I incorporated test cases written in an earlier fork, linked by @ppolewicz in https://github.com/slezica/python-frozendict/issues/14, but I'm torn between two positions: on one hand, the cases are pretty basic and not much is being actually tested, but on the other hand, most of the real action happens inside dict and OrderedDict, both provided by the stdlib.

I wanted to know your opinions before I merged this in. If you have time, please take a look!

[eugene-eeo]

While stdlib's unittest module is quite good, maybe we should use pytest or nosetests as they allow us to write more concise tests. Also we can package them in tests_require attribute so that normal installs don't install the whole test library.

[AlJohri]

can this be merged?