NpgsqlCommand.AppendCommandReplacingParameterValues() and operators @@, @> and <@.

Fix bug that would turn operators @@, @> and <@ in ::, :>, and <: respectively.
Add tests for said operator butchery.
NpgsqlCommand.AppendCommandReplacingParameterValues()

Remove a couple bits of redundant and ineffective code in the scanner.
Merge pull request #135 from glenebob/fulltext_operator_fix

Fix for operators @@, @> and <@.