Smoother orbit controls

This is a set of three patches that makes OrbitControls feel more fluid and physical, at least on my three Mac OS browsers (Safari, Chrome, FireFox).

These three patches do the following:
* Avoids gimbal lock, or east-west sticking, when at the North or South poles by setting more reasonable default minimum and maximum polar angles
* Adds some damping to the rotation controls (inspired by `TrackballControls`)
* Fine-tunes the dollying behavior by moving linearly, rather than exponentially, in distance. Feels much better on Apple trackpad devices.