Provide dynamic loading of modules

This adds a new interface to Luigi that takes a module parameter on the command line

Eg. you can run
python something.py --module my_luigi_job FooTask --x 1 --y 2

Inspired by discussion in #365
Merge pull request #378 from spotify/erikbern/dynamic-module-loading

Provide dynamic loading of modules
Add a script to find all deps on the paths b/w task T and task S.
This is a practical way of finding all upstream tasks of task S