Backpack Connect now merges its own querystring args into the callback URL's.

This is done instead of a raw append, which effectively prohibits callback
URLs from using their own querystring args.
consolidated private fullUrl() in backpack connect w/ utils.fullUrl().

Also added tests for utils.fullUrl().
Merge pull request #708 from toolness/querystring-merge

Backpack Connect now merges its own querystring args into the callback's.