import channel.pp and rspec tests

this allows subscribing to redhat channels via the defined type
spacewalk::channel. it also supports custom channels with custom pgp
keys used for signing.

you also need to specify a username/password for accessing the satellite
server. subscribing to channels works with an unprivileged user, so you
should create one.

i've also included rspec tests for the defined type spacewalk::channel.
import .fixtures.yml and a Rakefile for rspec
tests/init.pp was missing for running puppet apply

this allows module testing with puppet apply tests/init.pp.
Merge pull request #1 from tosmi/channel

add support to subscribe/unsubscribe to rhn channels