Implement lib/types.js.
Allow optional deep type checking.
Add initial tests of lib/types.js.
Eliminate lib/builder.js in favor of .builders export from lib/types.js.

Closes #2, finally!
Bump NPM version to 0.2.0 in preparation for publishing.

This is a big bump because the require("recast").builder interface was completely reimplemented, now with actual type checks, and there may be subtle differences in behavior.

One difference to note: Expressions will no longer be tolerated where a Statement is expected. Make sure to wrap such Expressions with builder.ExpressionStatement to keep the type checker happy.