Build rabbitmq-c without rabbitmq-codegen submodule

This is a partial fix for #96.

This patch allows building rabbitmq-c without having to do a git submodule init/update, which will users to download source archives from github that can be built out-of-the-box.

This also relieves the necessity to have python available.

It is still possible to regenerate amqp_framing.c/.h by passing REGENERATE_AMQP_FRAMING=ON to cmake or --enable-regen-amqp-framing to ./configure.