Allow (background) commands started with "&"

[elia]

With the current && implementation of commands it isn't possible to start commands ending with "&".

With this patch the following is possible:

tab 'Log (test)', :settings => 'Ocean' do
  run 'mate . &', 
      'gitx &', 
      'tail -f log/test.log'
end

I obviously use of the final "&" to speed up the whole process but it's probably only one of its possible uses.

[achiurizo]

thanks for the fix, your changes have been pulled!