Vector2, Vector3 and Rectangle ToString

Added missing ToString method for Vector2, Vector3 and Rectangle, so
they behave like their XNA counterparts
Vector4 ToString
Clamp method for Vectors

Added static methods for clamping Vector2, Vector3 and Vector4. Only
added methods that return values, was having trouble with methods that
pass values by reference and have out values (/* ref */)
Merge pull request #282 from nanexcool/vector2-tostring

Vectors ToString and Clamp