Add `assert_mocks` method, use Mocha's hooks with assert

This implements using Mocha's hooks with Assert and also adds an
`assert_mocks` method for calling the `mocha_verify` method. This
will ensure if any expectations are used, that they have been
verified. This should provide better mocha integration for assert
and allow using it correctly in an assert test suite.

Hooks.rb from Mocha:
https://github.com/freerange/mocha/blob/v0.13.2/lib/mocha/hooks.rb

Closes #2