Fix Issue With CSS Parsing

- This may be an issue with the ExCss library that was added to the
  repository, but for now I choose to fix it with this. The TermList
  AddTerm() method requires that each call to AddTerm() is separated by a
  call to AddSeparator(). If Parser.Parse() is called and there is only
  one css element in the passed string, then AddSeparator is not called.
  If a subsequent call is made to Parse() on that same instance of
  Parser, then it will attemp to call AddTerm() without calling
  AddSeparator(), causing an exception to be thrown. Thus, the solution
  here is to make sure that we use a new instance of the parser for every
  element that we are parsing.

@jameswelle