Setting up travis builds

##### Added .travis.yml
It does the following:
  * creates an avd
  * starts an emulator
  * waits for the emulator to start
  * sets up the android sdk and the required components (with the needed license)
  * builds the source code
  * uploads the built artifact (apk) to github releases (only if a git tag was created and pushed to origin)

##### NOTE
In order to get the upload of APK to work, we need to add an encrypted oauth token to .travis.yml. Because I do not have write permissions to nightscout/android-uploader, I'll need to work with someone who does to get their token encrypted and added to travis.yml. This can be done in a subsequent commit.