Add optional code auto-reload based on watchdog integration and forkloop from sauna.reload

If no one opposes, I'll merge fork loop -based code auto reloading into plone.act master soon. (Fork loop part is mostly based on code by @epeli for sauna.reload, but also he  has signed the contributor agreement.)

Reloading will be 'optional' and active only when plone.act is installed with [reload]. (It requires 'watchdog' for observing file system events.)

A bit outdated demo at http://www.youtube.com/watch?v=Dcrs1I1ytFQ (console output is much cleaner now).