Simple changes to the math package for convenience

[JesseTG]

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).

[nooone]

I think it makes reviewing PRs a lot easier if the one creating the PR writes a quick summary about what the changes are, why they were made and which use cases they cover.

With this quick overview in mind it is much easier for the reviewer to understand your changes.

[JesseTG]

Gotcha. Will do right now.

EDIT: Done.

[badlogic]

Cool, thanks!

[RotaruDan]

2 * (w + h) ?

[davebaol]

of course