Port some commits from from joyent/node@master

Ports a couple of commits over from joyent/node (master) that would be useful here.

Refs: https://github.com/nodejs/node/issues/17, https://github.com/nodejs/node/issues/21, https://github.com/nodejs/node/issues/31

7532d24 repl: Private Buffer object in lib/* files should probably have a test of some sort, or we should somehow check that removing the Buffer global doesn't cause issues in core somewhere else.

Chris suggested running Buffer = {} though a repl instance, but I'm pretty sure that would only catch the readline module without extensive use of other modules.

Generally, I'm still leaning towards making global.Buffer non-writable.