Implemented the first set of tests.

The tests are split into 4 test classes.

## `NUIButtonRendererTests`
The test class for the majority of properties.

## `NUIButtonRendererBackgroundColorTests`
A class which tests the setting of background colors.

## `NUIButtonRendererBackgroundImageTests`
A class which tests the setting of background images.

## `NUIButtonRendererForegroundImageTests`
A class which tests the setting of foreground images.

## `NUIButtonGradientSnapshotTest`
A subclass of `FBSnapshotTestCase` which checks that gradients are applied correctly.

I'm actually pretty relieved to find that the only test we actually need `FBSnapshotTestCase` for is gradients.

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I also took the liberty of adding a [Travis](https://travis-ci.org/) config file. Once this is merged in, a contributor or owner, must add the [GitHub webhook](http://docs.travis-ci.com/user/getting-started/#Step-two%3A-Activate-GitHub-Webhook).

The advantage of this is that any subsequent Pull Request will be run against the full suite of tests, and provide feedback whether it breaks the build/test config.

Let me know when this is done and I'll add the nice build status badge to the README.

Onto the next control...