Updated the post form

The date now uses the DatePicker form.

The main field now uses markdown editor only if the use_markdown checkbox is set to true.  Else it uses a standard text editor.  You can use a WSYWIG like TinyMCE by using the spree_rich_editor plugin and putting " post_body" at the end of the Rich Editor Identifications preference.

Signed-off-by: Nathan Lowrie <nate@finelineautomation.com>