Add require support

[jeaye]

Right now, jank reads single-file applications and it compiles clojure.core prior to the user code every time. Adding support for require will also entail the rest of the details around module loading and caching. This sets us up for being used within leiningen.

• support require
• make class path configurable
• add alias support

We won't be able to add the full ns macro until syntax quoting is done.

[jeaye]

Donezo!