Add Travis continuous integration support

Travis provides free [continuous integration](http://en.wikipedia.org/wiki/Continuous_integration) for public GitHub hosted projects. Sparkle would benefit from continuous integration to help avoid new code breaking old.

**After merging an administrator needs to complete the following steps to connect GitHub and Travis for Sparkle**

# Adding Travis Support

Travis requires administrator rights to the github repository. This is [not ideal](http://docs.travis-ci.com/user/github-oauth-scopes/) and a known limitation of GitHub's API.

To reduce the risk of problems and to allow security breaches to dealt with easily, isolate Travis support:

- Create a new GitHub user account called `sparkle-robot`
- Give `sparkle-robot` administrator rights to **only** the Sparkle code project.
- [Associate Sparkle with Travis](http://docs.travis-ci.com/user/getting-started/) through `sparkle-robot` 

In case of future problems, the `sparkle-robot` account can be isolated and administrator rights revoked.

## Adding Build Status

Once Travis has been set-up, a build status graphic can be embedded within Sparkle's read me file. The graphic will looking something like:

[![Build Status](https://travis-ci.org/DrawKit/DrawKit.png?branch=master)](https://travis-ci.org/DrawKit/DrawKit)

The markdown for a graphic like this is:

    [![Build Status](https://travis-ci.org/sparkle-project/Sparkle.png?branch=master)](https://travis-ci.org/sparkle-project/Sparkle)
We could use Travis's `xcodebuild` support but `make` keeps things loosely coupled and easily run locally.