Migrate to "watch" module for new file detection

[paton]

Fixes #8.

I did some light testing with this and things seem to work as expected, and new files are successfully detected and watched.

@nicjansma If you'd like to go this route, we can merge this instead of #7.

[paton]

For an explanation of the logic within compileOnChange, check out the example in the watch module readme:
https://github.com/mikeal/watch#watchwatchtreeroot-options-callback

[paton]

One issue with this: there's a (possible) bug in the watch module that causes the filter regex to only be applied on the initial walk, but not to new files, so if you create a non-handlebars file in the watched directory, it will trigger a do().

Just submitted a pull request to the watch module mikeal/watch#62

[nicjansma]

This looks great, thanks paton!