Add an .npmignore file.
Use recast.visit instead of recast.types.traverse.

Notice how the post-order traversal in lib/visit.js is now completely
trivial: all we have to do is call this.traverse(path) at the beginning of
the visitor methods.
Bump NPM version to 0.4.10 in preparation for publishing.
Support generator.return(argument) method for closing generators.

Closes #107.
Add a basic test of generator.return(argument);
Make sure generator.return(argument) works for newborn generators.