fixes RM7676 migrate -h doesn't produce help

also adds YARD doc to cmd_migrate in collusion with egypt.
low threat change, but still tested on Win7-32 sp0, ruby 1.9.3-p125,
Framework Version: 4.6.0-dev just for kicks
Merge remote-tracking branch 'kernelsmith/RM7676-migrate-h' into rapid7
Some whitespace and ruby -c fixes