mv the httpd service declaration into class apache::service

The service declaration should be in it's own class so that consumers of the
module can both require/depend on `Class['Apache']` and send notifications to
the Apache service.
Update dependency on concat to puppetlabs/concat
Merge pull request #317 from blkperl/concat_rename

Update dependency on concat to puppetlabs/concat
Add rspec-system tests for apache::service
Merge pull request #321 from hunner/service_class

Service class