A few 'mix escriptize' fixes:

* Check timestamps of .beam files before generating escript.
* Don't generate an escript if no .beam files exist.
* Add --force option to disregard timestamps.
* List command line options in docs.
* Correctly specify --no-compile as a switch to OptionParser.

Fixes #760.
Merge pull request #761 from alexrp/master

A few 'mix escriptize' fixes.