Dump Grunt in favor of a simple bash script.

This is better and faster for such a simple "build" IMO.
Add a `omitPrivatePaths` static for lean usage.

This also adds documentation for this function. There are better but
a little harder to implement approaches to this, but the basic idea is
that clients should be allowed to "privatize" lean objects - either
objects obtained with `document.toObject` or `query.lean()` (or other
more complicated cases... hehe).
Bump version to 0.4.0