Fix #75, SSHKit::Formatter.Dot prints dots

Change line 10 in lib/sshkit/formatters/dot.rb from `obj === SSHKit::Command` to `obj.is_a? SSHKit::Command` to better object comparision.
Add tests for SSHKit::Formatter.Dot
Test SSHKit::Formatter.Dot for passing commands and failing commands
Test SSHKit::Formatter.Dot for SSHKit::LogMessage
Fix #74, SSHKit::Configuration.format picks up on BlackHole

Change lib/sshkit/configuration.rb#L46 to pick up on constants that have more than one capitals by matching downcased format name and constant name.
Test ability to change format to `:BlackHole` (or `:blackhole`) as well as `:dot`