Add a way of installing a single recipe

[superbobry]

Possible usage example:

$ brb install foobar
E: Recipe 'foobar' not found!
$ brb install ocamlgraph
<installing ocamlgraph with all the dependencies>

[superbobry]

This is mostly done, the only problem is -- 'brb' is not aware which recipes are already present in the '_dep' dir, so it always tries to reinstall all the packages, required by the recipe.