add try/catch block around call to stats()
StatsTube shows stats for all tube if no argument is given

If the 'tube' argument is omitted, the StatsTubeCommand will display statistics for all tubes in the given Pheanstalk
Small changes based on comments by @armetiz
Merge pull request #43 from rjbijl/tubestats

StatsTube shows stats for all tube if no argument is given