NOJIRA

Add convienence methods to allow for a signature to be checked
from purely variables.   (i.e. not pulling from a request object)
This is not yet used in Sakai - but useful other places that use 
the basiclti-util code in a library.



git-svn-id: https://source.sakaiproject.org/svn/basiclti/trunk@125249 66ffb92e-73f9-0310-93c1-f5514f145a0a