Rename runtime/dev.js to runtime.js and get rid of runtime/min.js.
Revamp public API.

The public interface of Regenerator now more closely follows community
transpilation conventions:

  - require("regenerator") is a function that returns a writable
    stream for use with browserify -t (#20).

  - require("regenerator").compile is a function that transforms a string
    of source code and returns the result of recast.print.

  - require("regenerator").runtime() is now the preferred way to include
    the runtime.

  - The require("regenerator").runtime.{dev,min} properties are no longer
    exposed to client code.

I've been wanting to make these changes for a long time.
Bump NPM version to 0.6.0 in preparation for publishing.