Adding support of changelog templates to gitchangelog

This pull request adds the support of templates to ``gitchangelog`` through the of pystache, the python implementation of mustache, a logic-less template specification.

Templates are located in ``share/templates``. For the moment, only ReST and Markdown formats are available, but this should be pretty easy to add other formats.

In the README.rst I also added the description of the changelog data tree used to populated render the changelog. The elements are useful to know how to write a new template from scratch.