naked pkg support + major container refactor

`Cask::Installer` was already much too complex, so I took this
opportunity to throw a `Cask::Container` abstraction around the
extraction part of the package install step.

It goes like this: a Cask's URL points to a Container of some sort. The
containers we currently support are: dmg, zip, tar, and (new) naked.
Naked refers to a raw file that just needs to be copied in place. This
currently just means a pkg file, but in the future it may expand.

A Container knows how to do two things: identify a path as being its
type (`Container.me?`) and extracting the contents of its container to
the proper destination for a Cask (`Container#extract`).

The first Cask we have that supports the naked pkg type is
`heroku-toolbelt`. (Thanks to @sheerun for the Cask definition.)

Other miscellania batched in with this refactor:

 - switched to an explicit require strategy rather than globbing
 - `Cask::Installer` is instantiated now to match its interface with
   other similar collaorators
 - simplified zip and tar identification to shorter strings rather than
   exact matches of full `file -Izb` output
 - `Cask::SystemCommand` gets explicit output redirection options
 - many rogue backticks replaced to properly use `SystemCommand`
 - fixed misnamed test file `link_checker_spec.rb`
 - remove some extraneous `after` clauses in tests; leaning more on
   `test/support/cleanup.rb` to uninstall for us
 - pkg uninstall `:files` gets a `-rf` to support removing dirs

refs #839 and #1043