(fix) server doesn't return x_t, so this would never work

also, it wasn't even used
 (fix) Move timestamp into createAuthenticatedUrl from the constructor so that UserContexts don't become stale
Merge pull request #15 from omsmith/removeServerOffset

(fix) server doesn't return x_t, so this would never work
Merge pull-request #16 from jparker-d2l/timestamp-fix

Conflicts:
	lib/valence.js