Added Godot's math functions

[Zylann]

Added math functions from Godot, available in a Math:: namespace. Also added Math_TAU and improved a few things in vector classes.

Closes #107
Probably closes #385 too

[BastiaanOlij]

Looks fine to me,

[superRaptor911]

Template for lerp for float, double, Vector2, Vector 3 support?

template<class T>
T lerp(const T &minv, const T &maxv, float t) {
	return minv + t * (maxv - minv);
}