Add force polling option to wagon server

[uxtronaut]

This adds a force_polling (-o) option to the wagon serve command.

This was required for me to work locally on a wagon app being served from a mounted folder on a VM.

More discussion about using Listen over mounted file systems can be found at guard/listen#57

[coveralls]

Coverage decreased (-0.05%) to 75.19% when pulling 1ddf3c0 on uxtronaut:force-polling into 71670b2 on locomotivecms:master.

[johnallen3d]

Any chance this will be merged? We're running wagon inside of Docker containers for development purposes and experience this same issue.

[did]

better late than never...