Add util.format API.

It has the same placeholder with the util.format of node.js.
The callback function of replace is copied from node.js.
See https://github.com/joyent/node/blob/master/lib/util.js

And this patch also adds plugins/lib to plugin path.
Add console library.

The console has five methods(log, info, warn, debug and error).
Those method have to be implemented with variable length argument but
rhino cannot passes variable length argument to Scala native object,
because Scala implements variable length argument with Seq but rhino
doesn't know about Seq. So console passes one string converted with
JavaScript.