Camera boundry

I've seen this requested on the forum a handful of times, most recently here: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/z-kWiAZ1Jfg

For some applications, it would be helpful to define a rectangle to restrict the camera to view only a certain span of the map.

The issue names no repository files, tests, or entry points; begin by locating Cesium's camera navigation and view-rectangle APIs, then review the linked forum discussion for intended behavior. Done means applications can define a geographic rectangle and the camera cannot view beyond that span, with behavior validated by appropriate tests.

Written by the indexing model from the issue text.