Added simple wrapper for Filterrific::ParamSet.new so that it can be instantiated with Filterrific.new.
Override ActionView's form_for to add filterrific magic when applied to a Filterrific object:

* set field name prefix to filterrific
* override html method
* set dom id to :filterrific_filter to trigger js observers
* set url to current URL so that form submits back to self
Updated javascript behaviour:

* rename observe_field plugin to filterrific namespace to avoid conflicts if an app already uses the plugin
* fixed invocation of event observers (dom ready)
* changed CSS selector class for periodically observed to filterrific namespace to avoid conflicts