Add concurrency argument to async.auto

[vergenzt]

Adds an argument to async.auto to specify maximum concurrency with which to run tasks.

Fixes #635.

[vergenzt]

The Travis build failed because of someone else's non-deterministic test! :( Mine (auto concurrency) passes...

...
✔ auto
✔ auto concurrency
✔ auto petrify
...

✖ queue pause with concurrency
AssertionError:
[ 'process 1',
  'timeout 0',
  'process 2',
  'timeout 0',
  'process 3',
  'timeout 400',
  'process 4',
  'timeout 400',
  'process 5',
  'timeout 500',
  'process 6',
  'timeout 500' ]
deepEqual
[ 'process 1',
  'timeout 100',
  'process 2',
  'timeout 100',
  'process 3',
  'timeout 400',
  'process 4',
  'timeout 400',
  'process 5',
  'timeout 500',
  'process 6',
  'timeout 500' ]
    at Object.same (/home/travis/build/caolan/async/node_modules/nodeunit/lib/types.js:83:39)
    at null._onTimeout (/home/travis/build/caolan/async/test/test-async.js:2570:14)
    at Timer.listOnTimeout [as ontimeout] (timers.js:112:15)

[vergenzt]

(FYI I amended the commit (only changed the timestamp) to force a Travis rebuild.)

[cryptoquick]

👍

[vergenzt]

Bump. :)

[cryptoquick]

It would be nice to get back to using mainline async again. I'm still on your fork, @vergenzt
Bump 😄

[aearly]

If you can fix the conflicts, I'll merge this.

[vergenzt]

@aearly Done, rebased to master. Let me know if you need anything else!

[aearly]

Thanks! I swapped the concurrency arg's position to always have the callback as the last arg.

[cryptoquick]

Awesome! 😸

[vergenzt]

Sweet. Thanks for merging. :-)