First pass at improving the public utils API.

This library is great, but the visitors specification is very undefined.  An issue that should be tackled sooner-than-later is the internally accessed utils module.   Currently, it is very unclear what helpers are available and the best way to access them.  It'd be great if they were broken down into modules that you could explicitly include.

I don't necessarily intend for this PR to land, but hopefully at least spark discussion around this.
This appears to just be testing the test code... Could you instead assert that `jstransform.utils` is the same as `require('jstransform/src/utils')` ?