HTML reports

Implement an avocado plugin that generates nice HTML reports to the users, so they quickly glance and understand results from their test jobs. This functionality requires the `pystache` [1] library, a python implementation of the `mustache` [2] template language. This PR also introduces a subpackage that will allow users to selectively install this plugin in their systems, rather than making it mandatory.

[1] https://github.com/defunkt/pystache
[2] http://mustache.github.io/

![screenshot from 2014-11-24 00 44 18](https://cloud.githubusercontent.com/assets/296807/5160433/4c21aa20-7373-11e4-816c-95898c8de289.png)