Major code cleanup

- Added Docblocks _everywhere_
- PSR fixes
- Classes referred to by their fully qualified names. 
- Restructure folder to the Composer packages "norm" (ie. src/ and tests/)
- Replaced calls to `Carbon::` and `self::` to `static::` calls so that Carbon can be easily extended
- Made Travis run the PHP 5.5 tests too now that they're available
- Added quick test for `timezone()` and `tz()` – welcome to 100% code coverage

I mean I love Carbon but, the code was kind of messy.