Added support to bind a TaxonomyField to a specific Term, and relative test

Created new overloads for extension methods CreateTaxonomyField accepting a Term.
Created new private extension methods CreateTaxonomyFieldInternal accepting a TaxonomyItem, called by both TermSet and Term public overloads.

Created new overloads for extension methods WireUpTaxonomyField accepting a Term
Created new private (not extension) method WireUpTaxonomyFieldInternal accepting a TaxonomyItem, and being used by other WireUpTaxonomyField methods, for both Web and List fields.

Created specific unit test for creating a new TaxonomyField bound to a specific anchor Term