handle slide transition and datetime format on frontend

This is ready to merge once someone reviews it.

To test the transitions: add a field config entry of `transition` with one of the following values: slide, replace, *fade* (default).  This fixes #649.

To test the datetime formatting: add a field config entry of `format` with the desired format specification.  The default is the *localized medium date and short time* (this hasn't changed).  This fixes #397.

Format specifiers can be found here:
http://docs.closure-library.googlecode.com/git/closure_goog_i18n_datetimeformat.js.source.html#line32