Killable doesn't kill about coop

[deckarep]

I just ran into this issue: Here is a snippet demonstrating the issue: http://play.golang.org/p/XI5KfeNfpj

Not only does it not truly kill the function but the goroutine's are going to leak if the function is spinning in a busy loop.

Btw, great job on this lib and the Boom library!

[rakyll]

Since a goroutine can only kill itself by polling from a channel, it's not possible to break a block such as fn and return immediately. What Killable could do is maybe iterate over an input channel and kill when a signal is sent, which is a trivial sample. I guess I should better remove killable and timeout samples.

[rakyll]

Removed Killable.