aria2 plugin

Plugin for aria2, the "ultra fast download utility".

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-in
terface) 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.m
d).

(Note I added as a plugin in the main plugins directory since it is
[eventually] intended to be both an input plugin and an output plugin.)
Catch socket errors
Merge branch 'aria2' into develop