Block by default when calling start or stop

After @freerobby implemented the wonderful `block_until_running` and `block_until_stopped` methods, we've been using them everywhere in our tests. We've been using them so much, that it's hard to think of a time when you wouldn't want that behavior.

This bumps to version 1.0.0 and makes `NsqCluster.new`, `ProcessWrapper#start`, and `ProcessWrapper#stop` all block until the processes are started or stopped.

Optionally, you can pass `async: true` as an argument to any of those functions to skip the blocking step if you like.

What do you think @freerobby and @jamesfzhang?