[V2] HTML reports

This is a follow up to PR #262.

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/296807/5174334/7d554404-7417-11e4-929f-d224b59c14fc.png)

Changes from V1 - Implemented @clebergnu's suggestions, mainly:
 * Turned the function that gets plugins resource files into a plugin API
 * Used the `webbrowser` module to open the web browser.
 * If the user passes `-` to the HTML plugin, he/she will get an error. The reason is that we can't embed all resources necessary to proper viewing of the HTML on a single file.
 * Use relative links on the report that goes in the job report directory, absolute on reports in custom paths