Upgrade mocha to 0.14.0
Upgrade exvo_helpers to 0.6 line
Add 'rack' to dependencies

It was an implicit dependency, now it's explicit.
Store query params in `user_return_to` session param

i.e., when accessing a page - requring authentication - with query params
(like `/inbox?q=query`), after signing in, the query params would not
be preserved (they would be stripped). Now it will properly redirect the
user to the page with query params preserved.
Merge pull request #11 from Exvo/store-query-parameters

Store query parameters when doing redirect to auth