Add Travis CI support

Tests most aspects of the library at every commit and pull request.

After merge requires activation [here](https://travis-ci.org/profile/ShareKit).

Sample output for this pull request [here](https://travis-ci.org/rivera-ernesto/ShareKit). Fails because of pending pull requests and failing unit tests.

Supersedes #912.
`pod spec lint` doesn't lint agains the current commit but downloads another copy of whatever version the podspec points to.