Support for Mocha-style optional async beforeEach functions.

If the beforeEach function accepts one parameter, Dalek
will pass a callback function which must be called to signal
end of beforeEach phase.

If beforeEach takes no parameter, it behaves synchronously
as originally implemented.
s/asume/assume
Mocha-style async support for afterEach() functions.

If your afterEach() function takes a single parameter,
dalek will assume it is async and pass a callback which
you can call to indicate your afterEach() function is finished.
fix lint
Merge pull request #127 from niallo/async_each

core(unit): Optional support for async beforeEach() and afterEach()