first pass at scbMetabox class.

props @chacha for initial code
rename edit_form_advanced() to standalone() and make it final
mark some more methods as final
rename form() to form_fields()

form() is too ambiguous
move all optional parameters to an associative array
need to call add_meta_box() only once, for the current screen
improve explanation for scbBoxesPage::_intermediate_callback()
rename scbMetabox to scbPostMetabox
remove 'standalone' context

A metabox that doesn't have a title, isn't dragable nor hideable isn't
much of a metabox and it dilutes the focus of the scbPostMetabox class.
remove unnecessary temporary variable
Merge pull request #18 from scribu/metabox

Add metabox class