Use Testify package for test assertions

Prompted by @jingweno's [comment][], I whipped up a PR to swap out the bmizerany/assert testing package for [stretchr/testify][testify]. I like testify because:

- It is actively maintained.
- It has a number of additional [assertions][].
- It can provide a custom failure message without changing assertion method (no more `assert.Tf`).
- It has better formatting of failure messages.

Before:

```
❯ go test ./...
?       github.com/octokit/go-octokit/examples  [no test files]
--- FAIL: TestClientRel (0.00 seconds)
        assert.go:15: /Users/wynn/go/src/github.com/octokit/go-octokit/octokit/root_test.go:37
        assert.go:24: ! 1 != nil
 
FAIL
FAIL    github.com/octokit/go-octokit/octokit   0.049s
```

After:

```
❯ go test ./...
?       github.com/octokit/go-octokit/examples  [no test files]
--- FAIL: TestClientRel (0.00 seconds)
        Location:       root_test.go:37
        Error:          Not equal: 1 (expected)
                                != <nil> (actual)
 
 
FAIL
FAIL    github.com/octokit/go-octokit/octokit   0.048s
```

I made a pass over the tests and made some updates where one of the new assertions made the tests easier to grok.

[testify]: https://github.com/stretchr/testify
[comment]: https://github.com/pengwynn/flint/pull/16#discussion_r21583865
[assertions]: http://godoc.org/github.com/stretchr/testify/assert#hdr-Assertions