Allow RichTextWidget to accept TinyMCE init options 

The new ``options`` attribute allows one to configure any arbitrary init options for TinyMCE -- eg any option from http://www.tinymce.com/wiki.php/Configuration.

Option is specified as either a dict-style (or two-tuple) structure of Python values and converted into JSON.  These options are included in the JavaScript template for the widget against the TinyMCE init call.

My only suggestion beyond what's here is whether the existing TinyMCE-specific options on the widget (height, width, theme, skin) should be made to be defined as part of the `options` dict.  Having it all as one structure is more consistent (eg all options can be set in the same place) and makes the template easier to maintain (eg just a set of 'options' to output rather than 4+ individual settings to specify and name).  Backwards compatibility can be maintained by falling back to look for these 4 options where they currently exist as attributes on the widget.  Thoughts?