adding ability to silence log output

- Added a method `silence(<bool>)` to log module
- Wrapped log methods, `info`, `warn`, `error` and `sql` in conditional
  which disables logging if `this.isSilent === true`, unless
  `global.verbose === true`. Verbosity wins!
Merge branch 'silence-logging' of github.com:noazark/node-db-migrate into noazark-silence-logging