add Calibre Companion plugin

This PR relies on the merge of koreader/koreader-base#224.

This plugin implements a simple Calibre Companion protocol that
communicates with Calibre Wireless Server from which users can
send documents to Koreader devices directly via WIFI connection.

Note that Calibre Companion(CC) is a trade mark held by MultiPie Ltd.
The Android app Calibre Companion provided by MultiPie is closed-source.
This plugin only implements a subset function of CC according to the
open-source smart device driver from Calibre source tree.

More details can be found at calibre/devices/smart_device_app/driver.py.