Add patching to builds

This isn't ready for merging, but I'd like to discuss if my approach is acceptable.

Ruby-build and `rbenv install` supports [patch ruby](https://github.com/sstephenson/ruby-build#applying-patches-to-ruby-before-compiling) before building it.

I'd like to make use of this in the rbenv cookbook as I've got a couple of rubies that are failing to build unless a patch is applied.

This PR would add support for supplying patch files or urls.

As it current stands it:

* adds a dependency on `curl`
* has only been tested with user install (and it missing proper test support)

What do you think about this? Do you think using a separate file and url attribute is the right approach. Are you happy with a curl dependency being added? (I'll check for a curl community cookbook).