Mcquin/transfer trusted certs

This handles the second item on this list https://github.com/opscode/chef/issues/1542#issuecomment-53341384 in enabling SSL verification by default.

If `:trusted_certs_dir` is provided in your knife config, it will pull all the .pem and .crt files from that directory and copy them to `/etc/chef/trusted_certs`.

\cc @opscode/client-engineers 

Questions:
* Is it okay to assume `:trusted_certs_dir` is single-level?
* Is it okay to assume it only contains .pem and .crt files?
* I chose to check for its presence only in the knife config since we're adding this to bootstrap. Is that sensible? Should it be done differently?