EJSON.stringify objects in sorted key order.

Thus if two objects are EJSON.equals, their serialization strings will
be equal also.

(This should mean that reactive-dict could now allow objects to be
used as the key for `equal`).

Note this change does not affect livedata because livedata uses the
EJSON adjust functions to convert between JSON and EJSON (not
stringify).

Adds a couple options to EJSON.stringify:

* keyOrderSensitive: in parallel with EJSON.equals, outputs objects in
  native key order (the old behavior).

* indent: pretty-prints the output.

The serialization code is based json2, which makes it easy to preserve
the indentation functionality provided by JSON.stringify.  Only enough
code is included to serialize arrays and objects; serializing
primitive values such as strings is delegated to the native
JSON.stringify to reduce code and in case the native implementation is
more efficient.

Include underscore as an explicit dependency in package.js.

Document options to EJSON.equals and EJSON.stringify.