Update project + Travis CI

General project updates and Travis CI support

- Update all project dependencies (and target sdk level to 20)
- Add `gradlew` (this *should* be checked in to version control, see the [second paragraph of the doc](http://www.gradle.org/docs/current/userguide/gradle_wrapper.html)
- Fix some dependency issues, and add instructions on how to locally deploy youtube-android-player-api

To get Travis CI working: Go to https://travis-ci.org/ and sign in with your GitHub credentials. Then, go to your profile (on the Travis CI website) and enable CI for DraggablePanel.

I already added a build status indicator to README.md, so if you enable Travis CI **before** merging this PR, you should get the first build right away. :)