added support for 'form' args array to specify form action, method, ID, etc.
aded support for 'orderby_values' parameter in the orderby field
updated orderby processing
modified field methods to return rather than echo
add support for value ranges with meta_key fields
added 'default' parameter for fields
add private method build_form to first generate the search form without displaying it.  changed the_form to simply output that form.  allows form to be placed anywhere on page.  Also added: pagination fix.