do not change "( )" to "rdf:nil"

In tests/turtle:
rapper -i turtle -o turtle test-08.ttl
changes "( )" to "rdf:nil".
This change to src/raptor_serialize_turtle.c will produce "( )" in the
output file.
add tests for rdf-nil

test-08.ttl is valid test, but output is not saved => no change
for test of exact syntax, add files test-39.ttl and test-39-out.ttl
modify Makefile.am for these two new files
non staged changes are just dos2unix conversions
Merge pull request #16 from rhmccullough/rdf-nil

do not change "( )" to "rdf:nil"