Fixes #43, #44, #45

Closes #43:

* Use `brew cask list` on the cask to determine if it is installed. If
 it not, the command returns a non-zero exit status. The `exitstatus
 == 0` will return true or false, so we don't need an `if` block.
* Additionally, the `not_if` aren't required since we are only
 configuring the resource if `@cask.casked` is `false`.

Closes #44:

* Add `:install` and `:uninstall` actions. For compatibility, `:cask`
 and `:uncask` use these respectively.

Closes #45:

* Add `default_action` method to set the default action if it's defined.
* Print warning messages that Chef versions less than 10.10, when
 `default_action` was introduced, will be unsupported in the next
 major release of the cookbook.
I think the "public" API should be install and uninstall, since those are the actual commands for `brew cask`.