Increase assert test coverage from 86.6% to 97.1%, fixed bugs.

Extend tests for NotEqual, Len and add tests for Condition, InEpsilon.
Add tests for Assertion.Condition, Assertion.InEpsilon, Assertion.InDelta
Fix a bug for Assertion.InDelta, Assertion.InEpsilon having incorrect signature.
Minor doc string update
Merge pull request #76 from victorkryukov/master

Increase testify/assert test coverage from 86.6% to 97.1%, fix bugs.