fix #670 refresh code style

fix #670 

I tried to add most of the [Android Code Style Guidelines](https://source.android.com/source/code-style.html) in the checkstyle config file `cq-configs/checkstyle/checkstyle.xml` with a few differences:
 * max 120 characters / line (instead of 100)
 * use standard Java Guidelines for undefined styles (whitespaces, empty lines for instance)

New gradle task `gradle checkstyle` that generates a style report in `open build/reports/checkstyle/checkstyle-result.html`

New script to check for new style errors in the current branch (You need to install `checkstyle`, on a mac with homebrew: `$ brew install checkstyle`):
`$ tools/compare-with-develop-style.sh`
It runs checkstyle on the current branch and on the develop branch and notify you if new style errors have been introduced. That should ideally be ran before creating a pull request.