Bundle upgrade_helper plugin with jetpants core

This is a plugin to aid in performing a major MySQL upgrade, such as from 5.1 to 5.6. It includes the following functionality:

* New Jetpants command to clone a lower-version standby slave to a higher-version spare, and perform the upgrade process on the spare
* New Jetpants commands to do a master promotion that evicts the final lower-version node from a pool. You can choose to do either a standard locking master promotion (with read_only period), or on shards you may do a promotion that "ejects" the old master in a multi-step process without any locking.
* New Jetpants commands that wrap two Percona Toolkit tools (pt-upgrade and pt-query-digest) to verify the upgrade process did not cause any data drift or major performance degradations.