Added task to execute arbitrary commands

[Toflar]

This is a PR for #336.

Usage:

- exec { cmd: 'my command' }

If you like, you can pass a custom description like so:

- exec { cmd: 'my command', descr: 'My command does this and that' }

Also you can jail it (which forces remote execution) and adjust the timeout. 😎

[coveralls]

Coverage remained the same at 100.0% when pulling 50354a3 on Toflar:exec-task into b36f90e on andres-montanez:nostromo.

[andres-montanez]

Hi, tanks for the PR. I've tweaked it a bit.
Cheers!