Pull the plug on the default admin-party

Guest access is now disabled by default. Get off my lawn, you punks!
Pro tip: For testing, use the examples/admin_party.json config file to
start a Walrus server in admin-party mode.
Make default server (-url) be "walrus:"

This way you can launch the gateway with no arguments and it'll start
up, with no external dependencies. "Simple things should be simple."
To get the old behavior, add the args -url http://localhost:8091
Fix panic/crash in _changes feed (due to JS 'undefined' value)

The channels view was accidentally emitting a JS `undefined` value
instead of `false`, and Otto was returning a bogus result when
converting that back to a Go value, which then caused a panic.
Fixed the root cause by making sure false gets emitted instead.
Fixes #89.
Update submodules (otto and gomemcached) to latest