Add bower checker, update travis to use it and add separate testing script

[icyrockcom]

Continuation of #72

• Add dependency checker to the build which would confirm that all exercises are using the same versions as the common bower.json
• Update documentation for contributing to mention the additional restrictions
• Adds test-separate.sh that can be used to compile and test all exercises separately, instead of merging them like test.sh used in Travis build does. Had it built for previous testing, so figured I can include it. Might be useful down the road...

This completes things mentioned in #71.

@lpil please check if all good when you get a chance.

[lpil]

Great, thanks!

[icyrockcom]

Thanks @lpil!