Make underscore and async globals configurable

This should respect the flag in the default configuration file for globalizing `underscore` and `async`.

It also fixes the flag in waterline for globalizing models. It just needed to be moved under the collections key.

As we discussed in #295 the sails global can't be removed without breaking the controller conventions currently in use. 

I didn't do anything regarding the global adapters setting because it doesn't look like its used. The adapters are attached to the sails global but the option exists in the default config file.

This should fix #299 but you may want to go through and ensure it doesn't break anything you guys are using in production apps. Let me know if you see any issues.