Fix bug in length check in highly overlapping case

If one read is exactly the length of the overlap, the pair would be
scored using the wrong formula.
Improve the testing harness

This version is more powerful since you can create an arbitrary
assembler without having to redefine the header files.
Added documentation about adding a new algorithm
Autoformatted testing Vala code
Make sure testing library builds against the correct headers
Allow supressing quality diffs in testing

Useful for comparing different algorithms
Minor fixups to manual page