Support the login_hint parameter

Add support for setting the login_hint parameter which can simplify the authorization flow if the application knows the email address or domain the user should authenticate with. The parameter is defined at: https://developers.google.com/accounts/docs/OAuth2WebServer#formingtheurl
move login_hint to end
Clone headers before setting user-agent to preserve the semantics of httplib2 request method
Merge pull request #18 from jay0lee/patch-1

Support the login_hint parameter
Merge pull request #19 from uguatel/master

Clone headers before setting user-agent to preserve the semantics of httplib2 request method