BUGFIX: #13 wrong type, typmod identification when value was assigned to record field

This fix a issue reported by mscottie - false performance warnings in triggers.

The problem was little bit more common - type and typmod identification was wrong
in check_target routine for PLPGSQL_DTYPE_RECFIELD. Fiexed + new tests.
remove warning about unused function check_element. Add a check of valid types when array element is a target of assignment.
update example of build via `make` - now USE_PGXS is default, actualize copyright