Optional behavior to ignore message history if required

[jonfreedman]

Some xmpp implementations seem to ignore the the clients request to suppress message history

Note: It is known that not all service implementations support MUC history management, so in practice a client might not be able to depend on receiving only the history that it has requested.

Introducing a new HUBOT_XMPP_UUID_ON_JOIN variable which enables the following behaviour

• on joining a room the bot will send a uuid
• any messages are ignored until the uuid is heard

See https://github.com/markstory/hubot-xmpp/issues/104

[markstory]

Thanks. Tagged 0.2.3.