Allow blank Set fields

Changed XSD for Set field to include "" in the enumeration.  (Analogous to Compound having minOccurs=0)
Email fields can have leading/trailing whitespace

Email MetaFields now allow for (and trims) leading and trailing whitespace.
pass 'merge' and 'match' to render_search_value

MetaFields have access to 'merge' and 'match' values inside render_search_value
MetaField::Text message no longer ignores "small" words if match is 'EX'