Set status code to 504
Add 403 response
Add P-Charging* headers to incoming requests
Update terminating conditions
Merge pull request #454 from Metaswitch/issue443

[No reviewer] Update terminating conditions
Merge branch 'master' of github.com:Metaswitch/sprout into p_charging_headers
Updates after review
Merge pull request #452 from Metaswitch/p_charging_headers

[Reviewer: Andy] Add P-Charging* headers to incoming requests
Get status text from code
Merge pull request #448 from Metaswitch/iss402

[Reviewer: Mike] Set status code to 504
Use error code in subscription.cpp
Update comment
Merge pull request #457 from Metaswitch/http_error_code

[Reviewer: Rob] Use error code in subscription.cpp
Merge branch 'master' of github.com:Metaswitch/sprout into sto536_dereg