Speed improvement for the "lein help" command.

@RyanTM and I reduced the run time of the `lein help` command by making it just read the task source code files instead of actually requiring/compiling them.

This results in a 70% speed improvement on my machine (14 seconds before, 4 seconds after).

This pull request is messy because bultitude doesn't support retrieving the namespace forms from files.  The code added to `help.clj` was mostly copied from bultitude, so it should probably be refactored and moved back to bultitude.

If you like the direction this is going, let me know because I would be willing to make pull requests to bultitude and Leiningen to clean this up.