Execute tests using setup.py test.
Updated outdated README.
Initial implementation of path dampening.
Implemented Path.get_weight()
Assert sensible min_ttl value.
Work on Path tests.

* Mixins for trivial, dual and complex paths.
* Tests for path weight.
* Tests for path weight cache.
Remove debug statement.
Small cleanup of assertions.
Test for complex path with unequal scores.
Updated TODO in README.
Usage dampening from Graph in tests.
Implemented Ensemble data structure and weight.