#12 Implement EDG WP-07 extensions to HTTP

[Conrad Steenberg]

See
http://www.uscms.org/SoftwareComputing/Grid/VO/doc/datagrid.pdf

Extract:

By implementing backwards and forwards compatibility,
we are able to continue using https:// URLs.
Client request: GET-PROXY-REQ /request/uri/... HTTP/1.1
Delegation-ID: AUniqueString
Server response: 200 OK
Content-type:
application/x-x509-user-cert-request
PEM encoded cert request.
Client request: PUT-PROXY-CERT /request/uri/... HTTP/1.1
Delegation-ID: AUniqueString
Content-type:
application/x-x509-user-cert-chain
PEM encoded certificate chain.
Server response: 200 OK
Client request: GET /request/uri/... HTTP/1.1
Delegation-ID: AUniqueString
... etc as normal