add barrier.abort (issue #8)

[fkuo]

[davepacheco]

Thanks for doing this! Can you add some tests, an example, and update README.md? My main question is what the expected behavior is when you start an operation on a barrier that's aborted? The example would help clarify that.

Also, is "drain" the right event for conveying the error?

CC'ing @jclulow, since he's interested in this.

[jclulow]

If there's a different event for conveying errors, e.g. "error", then it would be good to ensure that "drain" does not ever fire in the error case.