bgp: add bgpspaker module for non Ryu application usage

This enables you to use Ryu BGP feature as 'bgp speaker' python
library, that is, without ryu-manager, RPC API, REST API, or other Ryu
stuff, you can use Ryu BGP feature in your python application (just
import bgpspeaker.py). The sample code and the API reference docs are
included too.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>