fixes imports in __init__.py and test_echo.py
changes deprecated assertEquals -> assertEqual
tests with assertRaises instead of using a flag
adds tests for exact number of failures
makes test helper closure python 2-compatible

Apparently `nonlocal` isn't a thing in Python 2, so you can't assign to things in an outer function scope -- but you _can_ mutate an attribute of the closing function. Implementation based on [this SO answer](http://stackoverflow.com/a/16032631/3408454).
Merge pull request #2 from mambocab/add-tests

Add tests