Update Salesforce.pm

Added logout() subroutine.
Ends the session for the logged-in user issuing the call. No arguments are needed.
Useful to avoid hitting the limit of ten open sessions per login.

See http://www.salesforce.com/us/developer/docs/api/Content/sforce_api_calls_logout.htm
Merge pull request #5 from redcharlie/patch-1

Update Salesforce.pm