Gitolite::Config::Repo objects should have more convenient methods for deleting/updating permissions

[wingrunr21]

Right now no functionality exists for easily updating/deleting permissions on an existing repo. This should be addressed. A proper DSL must be established since this functionality will have to deal with the fact that multiple perms of the same type can exist within a repo. Eg:

repo foobar
  R = bob joe sue
  - = sam
  R = jordan