OAuth signature should be verified against URL provided to the tool.

OAuth signature should be verified against URL provided to the tool. There is possibility that a proxy will change URL along the way (for example protocol from https to http) which would invalidate signature.

Details of the problem: 
  MITx course that launches next week is using LTI integration with lms.mitx.mit.edu. Their LTI integration correctly responds to the LTI parameters send their way, but when the request gets passed through the stack protocol on request gets changed from HTTPS to HTTP. This results in XModule failing to validate request signature.

The modification is to the code originally written by @cpennington. 

Timeline:
This is critical fix for MITx thus it has been hot fixed at MITx installation. I am not aware that it has same level of urgency for edX.org, thus merging them before December 2014 works for us.
@iceraj Is it possible to make try-except construct narrower in order to be more explicit about what we are checking?