CORS OPTIONS support for resources and items.
CORS Access-Control-Allow-Headers header.

Sent with CORS OPTIONS request. Added X_HEADERS configuration option.
Fix careless bug with CORS X_HEADERS response..
Fix PEP8 errors.
More PEP8 fixes
CORS OPTIONS; 'Access-Control-Allow-Headers' (Garrin Kimmell)
Garrin Kimmell
Merge branch 'garrinkimmell-cors-options' into develop