Non functional tweaks

Re#er comments added (remove namespace warning)
Extra test added
  Implemented overloads for IsEqualTo and IsEquivalentTo using custom comparers

Is.EqualTo and Is.EquivalentTo (for collections) now take a custom
comparer (IComparer or predicate)
Type safety increased in IsEquivalentTo

Replaced object with 'T' where possible to tighten type safety