This will be more accurate and avoid confusion that could arise if the self-reported "Number of Employees Being Sponsored" didn't match the number of confirmed contributors listed on the pledge.
Having that counted number stored in post meta is still useful, though, so this also adds/updates that number for a pledge whenever a Contributor post changes its status.
Fixes#48
Contributors associated with pledges have a state: they can be confirmed or unconfirmed. They also have some important meta data, namely when they were confirmed. Thus, managing contributor data for pledges is more robust if we treat them as their own post type instead of as a multidimensional array of post meta data.
This also reorganizes some of the functions related to pledges so that things are more consistent between the pledge CPT and the contributior CPT.
Fixes#11
