initial RSS 2.0 generation

This commit adds RSS 2.0 generation to harmonic.

It works by using a template located at ```bin/resources/rss.xml```. Since this template is tied to the harmonic installation it is not affected by changes in the users templates.

RSS files are generated for all languages.

A new ```harmonic.json``` key is used if present: ```author_email```. The RSS 2.0 spec requires/recommend that the managingEditor and webMaster channel entries should be emails. If this entry is present in the configuration file then it is used along with the ```author``` entry to generate something similar to **contato@andregarzia.com (Andre Garzia)** which is the preferred format described in the spec. If it is not present then just the author name is used. I doubt any feed reader will reject a feed because the those fields are not email.