Improvements to the Drupal Role > Membership Sync Module

There are two issues encountered by those who use the included civicrm_member_roles module. Please see this pull request for the code. https://github.com/civicrm/civicrm-drupal/pull/360

1. when choosing the 'cron' option, the script can 'time out' because of large numbers of members or sync rules, or both. Solution: allow an optional "limit" on the number of memberships sync'd per run, and sort the query by RAND when run for cron

2. some may wish to have the sync occur on in more than one of the listed situations, so the options have been converted to checkboxes instead of a radio button

Change #2 requires an upgrade plan for existing sites, as the way the data is stored has changed. We are prepared to discuss a best upgrade path with the core team, at your earliest convenience.