V0.6 wip

Hey folks,

Cask v0.6 is getting ready for release and I could use some help before it's completely ready:

* Try it out for your Emacs configuration
* Try it out for your Emacs packages
* Try out the new functionality:
  * The `files` directory and command
  * The linking functionality
  * The `build` command
  * The `package` command
  * Fetcher dependencies: `(depends-on "foo" :git "git@hosting.com/foo.git")`
* Check out the new public API (all functions below `;;;; Public API`).
  * Does the functions have good arguments?
  * Is the doc-string correct and easy to understand?

The tests do not pass on Emacs snapshot because of a bug in Emacs (see http://lists.gnu.org/archive/html/bug-gnu-emacs/2014-02/msg01569.html), but it should work mostly anyways.

Either make a comment in this pull request or make a new issue with your suggestions.

//cc @rdallasgray Heads up for Pallet!

Thanks!