Couple of usefull features for AnyEvent::FIFO

Hi

I found AnyEvent::FIFO module very useful and I've added couple of features that made my life a bit easier:

1) ->active, ->waiting to examine number of tasks active and waiting respectively.

2) Add an internal condvar to call ->begin and ->end, also ability to provide external condvar. Saves some typing in typical usage.

3) Use AE::postpone instead of AE::idle. AnyEvent's documentation specifies when AE::postpone is called.

There's couple of other minor cleanups (I committed changes for version 0.00002 which I could not find in your repo).

I would appreciate if you could let me know if you would accept those changes or if I should make some improvements before you could accept them.

Thank you!