Trackable projects

For a tool I wrote for tracking my hours I needed a list of all the projects/tasks I can track, and I finally got around to implementing it properly with a few specs. (My old implementation with its own API class and without specs can be seen [here](https://github.com/jvdp/harvested/commit/e2332c58d567d34a7e97013130253ac6b80ad78b).)

As it uses the 'daily' endpoint I put it under api/time.rb (with some refactoring). Please let me know what you think. I would like to use my tool with the mainline gem.