add plugin for hitbox.tv

here is a plugin for hitbox.tv - both live and vod are supported. i think i have supported multi qualities properly but there are not many streams to test with (most only have "best quality").

live example:
`livestreamer http://hitbox.tv/supergreatfriend live`

vod example:
`livestreamer http://www.hitbox.tv/video/10042 720p`

please let me know if there's anything i can do to improve it.