adding head option for formulae install

* references issue #9
* adds a little complexity in the recipe because type check
  is performed. I can't see this exploding in scope so don't
  feel too bad about the added 'if' statement
* the 'if' statement could check for String and handle the hash
  in the 'else' but wanted to explicitly call out the fact
  that we're looking for an ImmutableMash
* not sure what's going to happen when version 2 of hub becomes
  the normal version so this test is probably brittle
Merge pull request #10 from scottmuc/issue-9/install-head-version-of-formula

adding head option for formulae install