adding support for private repos on travis pro

Not sure if you have any interest in supporting travis pro accounts, but I've been using this locally for work stuff and its pretty handy. It just adds a check against the github api to see if the repo is public (no way to check with certainty if its private), and if its not, it prompts for the user's github credentials, and uses them to authenticate with travis.

```bash
Fetching build status for pwmckenna/private-repo:master

username: pwmckenna
password: superSecret

    ✔ pwmckenna/private-repo
        Compare:  https://github.com/pwmckenna/private-repo/compare/cf14ef3361a1...7d86b1aaa404
        ✔ 7d8c621 (master) 1.0.0 (Patrick Williams <pwmckenna@gmail.com>) (passed)
            ✔ 39.1 node_js 0.10.1
```