Added user agent to account-github's getIdentity() function. This is untested.

See https://github.com/meteor/meteor/issues/981
accounts-github: Generate user agent string from release version
Update History.md

Conflicts:
	History.md
Thank pmark
accounts-github: Also send user agent on oauth login call
0.6.2.1
Merge branch 'release-0.6.2.1'
Merge remote-tracking branch 'meteor/master' into win0.6.2