Allow arbitrary schemes within HttpResponseRedirect()

[zopieux]

redirect_uri is supposed to handle arbitrary schemes as per the RFC; this is especially useful for mobile app handlers eg. mobile-app://my/callback?code=....

To this end, this PR implements UnsafeHttpResponseRedirect in oauth2_provider/views/http.py that does the same thing as HttpResponseRedirect except it does not check the scheme, as it is already done in the various validators.
It's a shame there is no base class in django.http to do the Location header stuff without the check, but anyway.

A test has been added to check this bugfix.

See #154 and #155 for discussions regarding the validation of the redirect_uri field iself.

[coveralls]

Coverage increased (+0.01%) when pulling cb1f090 on Zopieux:fix-urlredirect-schemes into 060022f on evonove:master.

[zopieux]

One Travis job failed because of a Django1.6 tarball wrong checksum.

[synasius]

#155 it's now merged!