Add the beginnings of a proper plugin API.

For now it contains a way for plugins to load support
modules that are located in the same plugin directory
as the main plugin.
Split the justintv plugin into multiple parts.

justintv - Justin.tv only code
justintv_common - Code common between Justin.tv and Twitch
twitch - Twitch only code
plugins.justintv_common: Add a option to use legacy stream names.

The video height is available in the data we get
from Twitch, so we can easily replicate the old names.
cli: Notify when a new version of Livestreamer is available.
plugin: Force lowercase stream names.

Fixes #179.
cli: Allow case-insensitive stream name lookup.