src: enable --harmony by default

Harmony support is probably the most anticipated feature in v0.12.
Enable it by default so module authors can safely assume that Harmony
features like generators and proxies are available.  It's possible to
disable Harmony features again with the --no-harmony flag.

Update src/node_contextify.cc to make it stop copying properties from
the parent context indiscriminately.  It copies over the Proxy object
from the parent context but the new context has one of its own.

Update test/simple/test-repl.js; it had a test that checks for something
that's prohibited in ES5 strict mode but allowed in ES6, namely block
functions.

Suggested reviewers: I don't know, @indutny, @tjfontaine and @trevnorris?  I'm nothing if not inclusive.