Adds callbacks from dependent parent to parent formula, so that composite formulae can be built up that
compose multiple dependent formulae, with a completion notification after each child finishes install.

Adds support for taps to be declared within a parent formula.  Useful when a dependent formula requires
a tap.  Also adds a getter that allows the parent to suppress caveats, in the event that the parent
will issue a composite caveat.
Merge branch 'master' of github.com:dhallman/homebrew