Add `genome disk allocation purge` CLI

It requires a `reason` and a list of `Genome::Disk::Allocation` objects.
clarify help_detail to roughly describe purging
use sprintf style status messages
differentiate help for Purge and Deallocate
allowing user to input insert size params without being overridden
remove spurious 'my', simplify insert size checking
Merge branch 'master' of https://github.com/genome/genome into bsmap
Merge pull request #331 from mark-burnett/add-purge-command

Add `genome disk allocation purge` CLI
Merge pull request #339 from chrisamiller/bsmap

allowing user to input insert size params without being overridden
change methratio to 2.74
add methratio 2.6 and 2.74