Rewrite of CiviCase API

Here is my major overhaul of the Case API. It includes:
<ul><li>Dozens of bug fixes</li>
<li>Documentation of every function</li>
<li>Conforms to API standards</li>
<li>New search functionality supporting three search modes</li>
<li>Robust "get" functionality, returning all activities, contacts & roles as well as basic case info</li>
<li>And quite a bit of code cleanup</li></ol>