add waiting to status, as it is an essential part of the status

[orotemo]

The purpose: add the count of waiting processes to the status (queue:len/1). I believe this is an interesting part of a pool status.

this is a quick suggestion, and of course comments are welcome.
I realise it may break a lot of code involving poolboy:status/1 calls, so another call is one suggestion.

let me know what you think.

Thanks

[orotemo]

The probably better, easier solution is here