Allow homebrew to install tapped packages.

Homebrew has the concept of "taps": repositories with custom formulas.
These formulas live in /usr/local/Library/Taps, instead of the
usual /usr/local/Library/Formula.

This commit checks for the presence of tapped formulas, to allow for:

```ruby
  # Without this commit, babushka would be unable to find and install this package
  dep do "apple-gcc42", :template => "bin" do
    requires "homebrew-dupes.tap"
    installs "homebrew/dupes/apple-gcc42"
  end
```

Note taps still need to be installed before babushka will find
them:

```ruby
  dep "homebrew-dupes.tap" do
    requires "homebrew"

    TAPS_DIR = "/usr/local/Library/Taps"

    met? do
      "#{TAPS_DIR}/homebrew-dupes".p.exists?
    end

    meet do
      shell "mkdir -p #{TAPS_DIR}"
      log_shell "Brewing Homebrew Dupes tap", "homebrew tap
  homebrew/dupes"
    end
  end
```