Use Gradle dependency for Facebook SDK

This removes the source code integration of the Facebook SDK.
Merge pull request #10 from friederbluemle/use-facebook-dependency

Use Gradle dependency for Facebook SDK
Do not fail release build without signing config
Add default parameterless constructor
Fix Lint errors
Update Travis CI script
Merge pull request #11 from friederbluemle/fix-errors

Fix errors
Do not run connectedCheck on Travis CI
Fix whitespace and indentation
Do not use + in version numbers
Use Gradle dependency for Volley
Update Google Play services lib to 5.0.77
Update Dagger to 1.2.2
Merge pull request #12 from friederbluemle/update-dependencies

Update dependencies
update google play service version
Update Gradle wrapper to 2.1
Update Android Gradle plug-in to 0.13.2
Update repositories declarations
Update gms and gson dependencies
Merge pull request #13 from friederbluemle/update-project

Update project