Class parameters and parameter conflict detection

This is a reworked PR (a replacement for https://github.com/puppetlabs/puppet-dashboard/pull/142)  that contributes only

- class parameters support
- immediate conflict detection using ajax

Some contributions from the original PR have already been merged into master (jquery placeholders, rspec fixes). There's one more that has been excluded for now - the performance optimization (based on replacing OpenStruct objects with native ruby hashes). I can remake that change if it's demanded, probably after this PR is merged into master.