creddit: libreddit: Move UserAgent string into RedditState

This patch moves the currently used UserAgent string into the
RedditState. Origonally, it was just a defined macro to a constant
string in libreddit (Which doesn't really make any sense). This patch
allows it to be changed and set on runtime to whatever is wanted/needed.
libreddit: Add libreddit UserAgent

This patch adds a libreddit UserAgent to the full UserAgent string. Ex.
With cReddit, the full used UserAgent is:

"libreddit/0.0.1 (http://github.com/Cotix/cReddit) cReddit/0.0.1"