Merge pull request #7 from elentok/master

Added a check if the local port is already in use
Fix #6 - Use one configuration file instead of a directory.
Update README with example of one-file configuration
Move error message from local_port_used? method
Code cleanup
Rename list action -> active
Verify tunnel killing before continuing. Fix #2
Add an error message for older versions with dir-configs
Bump to version 0.7
Fix readme note about 'shaft list' -> 'shaft active'
Fix Readme note about config dir -> file