[BUGFIX] pm.max_children too small for some (XHR-heavy) projects

The default of 2 is a bit too small for most of our projects here, which sometimes require a few quick simultaneous connections per URL.  It looks like this update might be useful for others, too (e.g. boxen/puppet-php#46).  I don't think having a default of 10 will hurt anything, but am open to other suggestions/approaches (e.g. exposing the max_children setting at a higher level?)...