Added LIMIT/OFFSET support for PostgreSQL
Added support for "raw query"
Disable broken testFilterClass

It looks like Select::setFilterClass does not exist anymore.
Fix rendering when no filters are added

Calling $select->filter() and not adding any actual filters results in
invalid SELECT (WHERE part is without content). Filtering the array
prior returing fixes this.
Restrict Func()'s $args type and make it optional
Improve method chaining