Add unit tests and travis support.

@plasticboy please have a look an enable the Travis build if OK.

This should make the project much more maintainable.

The unit test library is: https://github.com/junegunn/vader.vim . It is likely the best pure vimscript one out there, and it is actively maintained.

cc. @ilikepi @shirosaki this is quite large, so let me notify you too =)