Issue #121: Move ColorSet out of Wallet Model and put it into coloredcoinlib

* refactored ColorSet to use colormap instead of model
* moved ColorSet into coloredcoinlib under colorset.py
* added unit tests for colorset.py in tests/test_colorset.py
* fixed a bug found from unit testing in colorset.py for get_earliest
* added coloredcoinlib/__init__.py so wallet_model isn't so verbose
Merge pull request #122 from jimmysong/121

Issue #121: Move ColorSet out of Wallet Model and put it into coloredcoi...