aria2 plugin

Plugin for aria2, the "ultra fast download utility". (See prior pull request [here](https://github.com/Flexget/Flexget/pull/188).)

Requires [aria2c](http://aria2.sourceforge.net) be installed and running in [daemon mode](http://aria2.sourceforge.net/manual/en/html/aria2c.html#cmdoption-D) with [XML-RPC](http://aria2.sourceforge.net/manual/en/html/aria2c.html#rpc-interface) enabled, which are both set in the aria2 config file.

Plugin documentation can be found [here](https://github.com/tubedogg/.flexget/blob/master/plugins/README.md).

(Note I added it as a plugin in the main plugins directory since it is [eventually] intended to be both an input plugin and an output plugin.)