update oarequest.js to support user stream events

all user stream events are namespaced under user_ for clarity.  
See twitter documentation for more details on their structure
https://dev.twitter.com/docs/streaming-apis/messages#User_stream_messages
change user stream event prefix from user_ to ustream_
update readme with documentation on ustream_ events
minor change to docs
add user_friends event

user stream specific event, documented here: https://dev.twitter.com/docs/streaming-apis/messages#User_stream_messages
update readme docs to document user_friends
Merge pull request #35 from damianb/user-stream-events

add support for user stream events