Patch web2py to return cross-domain exceptions

We've been losing some responses in cross-domain situations (esp. API
alls), since web2py throws away headers in some cases. The new
make_response_CORS_compliant method just restores liberal CORS headers
if they're missing.

N.B. We seem to be using different web2py versions on different boxes!
We should pin to a single version so that these patches don't wreak
havoc...