add Content-Type and Accept headers

According to the jsonrpc 1.2 spec, as well as the HTTP transport
proposal, good citizen implementations of jsonrpc are required to
include these headers. Servers "should" refuse clients not specifying
these headers as well.
properly handle nested methods

Previously nested method support ended at the first nested method,
since call would clear out the internally stored method_name. Instead
this approach creates a class returned from getattr, and properly
handles nested method calls. As a bonus it also denies access to private
methods as well. The tests are a bit contrived, but still valid
Better pep8 compliance
Add Matt Broadstone to the credits section
Make sure we don't overwrite user-defined headers when configuring RFC-defined ones
Merge http-headers
Test private methods protection mechanism