Feature ci improvement

Hi,

I'm opening this pull request so You can reflect and follow the progression.
What's done:
- Added Travis build status icon to readme, you can take a look at here how it looks like:
https://github.com/vpmedia/Starling-Framework/tree/feature-ci-improvement
- Merged gitignore files, so only one left in the root
- Added Gradle build files so Samples can be built automatically 
- Fixed Maven build script issues (checksums and config tweaks)
- Integrated FlexUnit into Travis build process, so other pull requests will be validated against tests