webidl-check: Harden against XSS.
webidl-check: Avoid bad parameter inputs.

There's two halves to this commit:

* Specify an action for the POST form.  This prevents values from previous
  GETs being carried over to new form submissions.

* Use the fields.getfirst() method to ensure we get exactly one value.
Merge pull request #12 from brettcs/audit-2014-02

Security fixes