velvet: option to support longer kmers

Velvet is usually built to support a maximum kmer size of either 31 or 63
and this patch adds a '--with-63mer' option to build the latter.
velvet: option to support longer kmers (alternate)

Uses the values of the MAXKMERLENGTH env. var. instead of a --with option.
velvet: bugfix: sha1 changed

The file http://www.ebi.ac.uk/~zerbino/velvet/velvet_1.2.08.tgz
was replaced with a different version on the server.
oases: new formula: Oases 0.2.08

A de novo transcriptome assembler for very short reads
Merge branch 'oases'
Merge branch 'velvet'