Add support for specifying custom request and response implementations.

I noticed that people using pecan have taken to writing custom webob req/resp
subclasses and monkeypatching onto `pecan.request` and `pecan.response`.  Let's
give them what they need to do this properly.

Change-Id: If0ac953e381cec3a744388000a3b3afc0ea2525c
Merge "Add support for specifying custom request and response implementations."