Update contributions.rst and add instructions for non-Python dependencies.

Include installation instruction for non-Python dependencies so newcomers to the project have an easier time setting up their environment.  Currently includes installation instructions for Ubuntu 14.04 / Debian-based environment.

* Fix installation instruction for Grunt.  The installation command was incomplete.
* Mention the Python3.4 and libffi header files required to install Warehouse.
* Add non-python-dependencies.rst, which currently contains instructions for installing / setting up non-Python dependencies in Ubuntu 14.04 or Debian-based environments.
* Link to document containing the installation instructions of non-Python dependencies.
* List the document containing non-Python dependencies installation instructions in the Table of Contents so that Warehouse documentation can be successfully generated by tox.