Adding a getUserId function

Adds a function to retrieve the _userID_ for the logged in user.

Note I have stripped the userID from the full value that is being reported back following the login. 
i.e. if _oauth.id_ = "https://login.salesforce.com/id/00DR0000000ABCDMAK/00520000001ABCoAAO" then _force.getUserId()_  returns _"00520000001ABCoAAO"_