Add example of installing packages with options

I found myself needing this often enough to think others might find the example useful when first reading the readme page, instead of needing to hunt through the issues to find out how to do this:

https://github.com/boxen/puppet-homebrew/issues/9
FormulaInstaller#install_bottle? is gone; update monkeypatch
re-add support for older homebrews
Merge pull request #37 from boxen/update-boxen-install-monkeypatch

FormulaInstaller#install_bottle? is gone; update monkeypatch
boxen-monkeypatches: fall back to default bottles.

Homebrew provides a bunch of binary packages for things now. When these
have `cellar :any` it means they can be used for installations outside
`/usr/local`.

This commit means that when there isn't a Boxen bottle available it'll
fall back to the Homebrew bottling infrastructure.
Merge pull request #38 from mikemcquaid/use-homebrew-bottles

boxen-monkeypatches: fall back to default bottles.
Tell which bucket is being used to download the package
Avoid adding more function inside Homebrew class

Even if we are prefixing the method it may clash with Homebrew own
method
Merge pull request #35 from mrchrisadams/patch-1

Add example of installing packages with options
Add a custom type/provider to manage homebrew

This allows us to require a minimum revision of homebrew, like so:

```puppet
homebrew_repo { '/opt/boxen/homebrew':
  min_revision => 'd34db33f',
}
```
Require a minimum homebrew revision

This revision is one that provides some functionality that
puppet-homebrew is going to need soon, so I chose it as the first rev to
depend on.
only run 'brew update' if your platform is Darwin
Merge pull request #43 from gorsuch/darwin-only

Only run 'brew update' if your platform is Darwin
Raise unless we're running in Boxen
You can't test for resources with underscores :(
Remove debug output
Merge remote-tracking branch 'refs/remotes/origin/master' into manage-homebrew-revision

Conflicts:
	manifests/repo.pp
Merge pull request #42 from boxen/manage-homebrew-revision

Manage homebrew revision
Handle minimum revisions which we haven't fetched

`git rev-list` returns a non-zero exit status if its asked about
a ref that it can't find (logically). We need to handle this case,
since the minimum revision specified may not have been fetched
from the remote yet.
Merge pull request #44 from boxen/handle-missing-revisions

Handle minimum revisions which we haven't fetched