Initial capsule build API (especially useful for plugins writers)

Hi Ron,

there are a couple of functions especially useful for Capsule plugin writers (egoistically speaking, https://github.com/circlespainter/lein-capsule, in my case especially helpful together with this https://github.com/cemerick/pomegranate/pull/69) I'd like to expose as public but they're part of a build-time Capsule API, which doesn't seem to exist yet (as far as I have seen).

In order to reuse some information from the main capsule module I did some refactoring there too (mainly exposing constants in `DependenciesManager` but also some stdlib-level Java utilities).

Do you think this could be merged or do you have any suggestions to improve it (or about other approaches)?

TIA! - Fabio