Initial ABRT proxy support

This change makes it possible for the proxy to pretend it is ABRT
(Automatic Bug Reporting Tool[1]) server. When it receives a problem
report, it forwards notification of the error to Foreman, and optionally
also forwards the original report to an actual ABRT server.

Non strictly ABRT-related changes:
- Move module Proxy::ChefProxy to Proxy::Request
- Add :foreman_ssl_ options to settings example

[1] https://github.com/abrt/abrt/wiki/ABRT-Project

For some notes regarding usage, see https://groups.google.com/forum/#!topic/foreman-dev/Yk2GrW9ne20

I am quite new to Ruby so please let me know if anything should be done in a different way:)