node: remove ev-emul.h

libev has been deprecated since v0.6. Add-on authors have had two major
release cycles to upgrade. Out it goes.
repl: allow overriding builtins

Don't give names of built-in libraries special treatment.
Changes the REPL's behavior from this:

  > var path = 42
  > path
  A different "path" already exists globally

To this:

  > var path = 42
  > path
  42

Fixes #4512.
Merge remote-tracking branch 'origin/v0.8'

Conflicts:
	configure
	deps/uv/src/unix/core.c
	deps/uv/test/test-fs-event.c
	src/node_version.h