Bump express-state dependency to v1.1.x
Leverage express-state's `cache` option to improve expose() performance

This updates how Express YUI exposes data to the client by using app-
scoped exposing and caching the result to increase the performance of
serialization.

A non-enumerable `__modified__` flag has been added to the YUI `config`
object which is used by the methods that modify the `config` to signal
that it seems to be re-exposed and re-serialized by express-state.
Bump express-state to ~1.1.1
Remove `__modified__` flag in favor or requiring developers to re-expose

Instead of trying to use a "magic" `__modified__` flag, it will be up
the the developer to re-expose the `YUI_config` if they make changes
to it after the first request to the app.
Merge branch 'master' of github.com:yahoo/express-yui into express-state-1.1.0

Conflicts:
	tests/units/middleware-test.js
Update API docs, README, and HISTORY for express-state cache change