Add ability to modify postcommit hooks on a bucket

Just like the precommit ability already included, this allows the modification of postcommit hooks on a bucket.  Configured hooks will appear in a `postcommit` property in the list returned from `rhc:get_bucket/2`, and they may be set by including a `postcommit` property in `rhc:set_bucket/3`.

If this PR is accepted, @rzezeski's PR #6 to add the `search` bucket property should probably also be accepted.