Support passing repository lists through from leiningen or standalone

[tomfaulhaber]

With commit c271a55 we switched to using pomegranate for grabbing dependencies. This is great, but it means that we need to pass a list of repositories to pomegranate. Right now, I'm just using the default list of clojars and central, but we need to extend this in two ways:

• Support a :repositories key in the params map that overrides the default (make sure that the default is also available there so that we can include it).
• When run under leiningen, take the :repositories value from the project.