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.
All of the following is specific to this environment so this header should reflect that the rest are subheadings of this section.