Multiple scopes

[fizker]

I made a small implementation of getting key('a', 'scope1, scope2', callback) to work.

Basically, that will be a convenience method for doing the following:

key('a', 'scope1', callback)
key('a', 'scope2', callback)

I also added a test for it.

I am well aware of issue #24, but it seems to implement the opposite, so I decided that a fresh pull request would be better.