Merge node-sass-cli into node-sass, for a better cli with watch.

- Using node-watch to watch directories or files, exposed with the
  --watch option
- Updated optimist dependency
- Split bin/node-sass into a lib/cli.js and bin/node-sass file, so the
  cli can be required (especially good for testing or extending)
Fix cli options and make the cli better testable with require()

Nothing is directly written to stdout/stderr but is passed in an event
emitter, so when testing we can use that.

This way of testing the cli has the advantage that we don't need to exec
for all tests.
Add some extra CLI tests
Merge pull request #147 from arian/improve-cli

Improve cli, merging node-sass-cli, which includes --watch