deform2: AutocompleteInputWidget - more power configuration for typeahead.js

This adds a ``datasets`` parameter to ``AutocompleteInputWidget`` which can be used as an alternatives to the ``values`` parameter for configuring the typeahead.js plugin.

Basically the value of ``datasets`` (if given) is JSONified and passed directly to the typeahead plugin.  In addition,
a custom JSONifier is used.  The JSONifier respects a ``literal_js`` marker class that can be used to include
hunks of literal javascript within the ``datasets`` parameter.

(A patch to deformdemo with a demo is forthcoming.)