Refactoring FASTA work to break contig sizes.

This change fixes #109. Specifically, on FASTA import, assemblies that were too long (>800kbp) would cause things to go a bit wonky. To resolve this, we break a single contig up into contig fragments. Additionally, this PR adds a few utility functions for manipulating contigs.

As a note, I've held off on editing the CHANGES.txt/md until there is consensus on which one we are sticking with.