Added minimal pyinvoke example (paver replacement)

I'm at the PyCon sprint working on migrating all tasks in **rakelib** to Python to eliminate the **rake** dependency. We currently use *paver* but it seems that it's not very well suited for defining hierarchical and interdependent tasks, so we decided looking at alternative solutions. I built a minimal example using **pyinvoke** (https://github.com/pyinvoke/invoke) which demonstrates how to define and run tasks there.

After installing pyinvoke (pip install invoke ; I didn't add it to requirements.txt since I couldn't locate it), you can get a list of all the tasks like this:
 
    #run in edx-platform
    invoke --list

    #this should print:
    Available tasks:

      quality.pylint.cms
      quality.pylint.common
      quality.pylint.lms


You can run individual tasks like you would in rake (except that you replace ":" with "."):

    #Runs pylint on all systems (cms,lms,common):
    invoke quality.pylint.all

    #Runs pylint on cms with arg foo = 'bar'
    invoke quality.pylint.cms --foo=bar

This is just a minimal example I want to put up for discussion, so please provide some feedback. Maybe the people working on the **paver** tasks can also comment on the difficulties they encountered so far, so that we can construct a more complete example in **pyinvoke** and see if it's a better fit.