Update scripting.md script package sample

Update the sample code for "Creating A Script Package" to use
`robot.load`. Previously it was duplicating the `exists`/`readdir` logic
that `robot.load` already has, except it was doing the exists check
asynchronously, which changes script load order and also means the
script won't have loaded in time when using the `--config-check` flag.
Update require('hubot') to also export Message in addition to its subclass
Merge pull request #837 from github/export-message

Update require('hubot') to also export Message in addition to its subclasses
Merge pull request #831 from kballard/patch-1

Update scripting.md script package sample
Add CHANGELOG for 2.11.0
Version bump to 2.11.0
Merge pull request #840 from github/release-2.11.0

Release 2.11.0
Adding filter middleware
Adding filter middleware; updated to latest Hubot