Remove unused line in writeUnsignedAPTag

Length should never be 0. So the deleted line should never be called.
One could argue it should raise an error, but what was left in there
was a remnant from a previous implementation which is just wrong.
Add initial version of writeSignedAP

Only one test added so far.
Refactor writeSignedAP and writeUnsignedAP

They were almost exact duplicates except for the Writer used to write
the tag.
Refactor writeSignedAPTag and writeUnsignedAPTag

They were almost identical except for the tag number. Refactor them to
use writeIntegralTag.
Finish tests for writeSignedAP
Merge pull request #3 from michaelgwelch/addAPWrites

Add writeSignedAP