Cleanup and expand client-side parameter parser

As part of the implementation of a [request for parameter hints](http://meta.stackoverflow.com/questions/213523/can-a-hint-be-added-to-a-data-explorer-query-parameter), the client-side parameter parser is now (hopefully) more sanely written.

Assuming this switch doesn't cause any problems that didn't come up during testing, the server-side parser should be rewritten in a similar fashion, and we can probably save the preparsed parameter information for good measure when displaying the read-only view.