Drastically reduced the redundancy in flixel.math

The amount of redundancy in the flixel.math package was downright ridicolous.

`FlxVector`
 * moved length to FlxPoint
 * removed radians, moved degrees to FlxPoint (now named angle). Flixel should be consistent about its use of angular measurement.
 * moved isZero() to FlxPoint()
 * moved EPSILON and EPSILON_SQUARED to FlxMath
 * removed radiansBetween() and degreesBetween() -> use FlxPoint#angleBetween()
 * removed dist() and distSquared() - there's FlxPoint#distanceTo()

`FlxRect`
 * removed containsFlxPoint()

`FlxAngle`
 * removed angleBetween(sprite1, sprite2) -> use sprite1.toPoint().angleBetween(sprite2.toPoint())
 * removed angleBetweenPoint(sprite, point) -> use sprite.toPoint().angleBetween(point)
 * removed angleBetweenMouse(object) -> use FlxG.mouse.toPoint().angleBetween(object)
 * removed angleBetweenTouch(object, touch) -> use object.toPoint().angleBetween(touch.toPoint())
 * removed getCartesianCoords() and getPolarCoords() - no longer necessary with FlxPoint length and angle properties

`FlxMath`
 * removed pointInCoordinates(), pointInRectangle(), pointInFlxRect() - use FlxPoint#inRect()
 * removed mouseInFlxRect() -> use FlxG.mouse.toPoint().inRect()
 * removed dotProduct() -> use FlxVector#dotProduct()
 * removed vectorLength() -> use FlxPoint#length()
 * removed getDistance(point1, point2) -> use point1.distanceTo(point2)
 * removed distanceBetween() / isDistanceWithin() functions -> the usual toPoint() trick

`FlxVelocity`
 * removed moveTowardsMouse() and -Touch, use moveTowardsPoint() + toPoint()
 * ^same with accelerateTowards