Add MainActivityTest and some changes and refactoring

[alejandrocq]

Issue #28

I have made this improvements:

• Update SDK and build tools versions.
• Update to Java 8 in order to use lambdas.
• Add MainActivityTest (interaction test) and the necessary libraries needed for testing.
• Changes and general refactoring in some parts of the code.

I have commited my .idea files too, but I think that you should remove that directory from git (usually is not a good idea to track IDE files, and also gradle files have almost all the information to get the developing environment ready).

I hope this is useful :).

[ashik94vc]

Hey, thanks a lot. Would you be able to add the idea files to gitignore too. I'll review your code tomorrow around morning and will merge it.

[alejandrocq]

Hey, thanks a lot. Would you be able to add the idea files to gitignore too. I'll review your code tomorrow around morning and will merge it.

You're welcome! :) Yes, I can do that this afternoon. I can do another PR if you prefer to merge this before.

[alejandrocq]

I have added two commits before the changes to remove .idea folder files from git, and also I have simplified the .gitignore file a bit ;).