add userAgent reporting

@bootstraponline and I went ahead and added the mvp of client reporting: adding info to the useragent on JSONWP requests.

just `author/libraryName/version`

Now appium reports the useragent string on session requests, so we can better debug from user-supplied logs.

see:
https://github.com/admc/wd/pull/342
 appium/ruby_lib@af6d44c