Implement a set of logging functions.

Within the context of an `on()`, one can now call `debug()`, `info()`,
`warn()`, `error()` and `fatal()`.

These pass an instance of `SSHKit::LogMessage` to the formatter. These
*LogMessage* instances respond to `#to_s` and are ignored by the *dor
formatter*, rendered prefixed with their verbosity level by the pretty
formatter.