Swap bmizerany/assert test library for stretchr/testify

This change swaps out the assert testing library for testify because:

- It's actively maintained.
- Has a number of additional assertion helpers.
- Can over provide custom failure message without changing assertion
  method.
- Better formatting of failure messages.
Use Error, NoError assertions for error checking
Use Len assertion for asserting counts in tests
Use assert.Contains for clearer tests
Merge pull request #71 from octokit/testify

Use Testify package for test assertions