Various convenience changes in Vector2, Sprite, Text, Rectangle, Vertex

Made the following changes:

* Constructors with a single parameter to all Vector2s (sets both components to the same value)
* XNA-like Vector2f*Vector2f (component multiplication)
* Right and Bottom properties to IntRect and FloatRect
* Sprite and Text constructors accepting a Vector2f parameter, the new position of the object
* Explicit "(Vertex) Vector2f" cast

I understand if perfect compliance to the C++ API is a priority, but I think that these changes can make working with SFML.Net easier.
This can also be removed if unwanted.