Simple changes to the math package for convenience

This is mostly for me to get my feet wet in contributing to LibGDX.  I haven't done many pull requests before.

Anyway, this PR is nothing huge.  It just does the following:

- Adds `equals()` and `hashCode()` to `Circle`, `Ellipse`, `GridPoint2`, `GridPoint3`, `Ray`, `Segment`, and `Sphere`.
- Added methods for circumference and area (and their 3D equivalents where applicable) to `Circle`, `Ellipse`, `Sphere`, and `Rectangle`
- Changed `Rectangle`'s uses of `NumberUtils.floatToIntBits()` to `NumberUtils.floatToRawIntBits()`.
- Added `len()` and `len2()` to `Segment`
- Added a constructor for `Ellipse` that accepts a `Circle` (not vice versa)

I don't believe this is a breaking change (sans the usual negligible probabilities that come with changing the hash function).