Overload operators +,-,*,/ in CCPoint

Following the discussion in https://github.com/cocos2d/cocos2d-x/pull/2502 , I implemented the proposed changes in this PR.

CCPoint are very often used as 2D vectors. Overloading basic operators in CCP would lead to a much cleaner, readable code when manipulating points.
This have already been implemented successfully in other Cocos projects such as cocos2d-xna.

This PR allow to add/subtract CCPoint as vectors:
```c++
CCPoint a(5, 10), b(6, 10);
c = a+b; // c == (11, 20)
c = a-b; // c == (-1, 0)
```
It also defines multiplication by a scalar:
```c++
c = a/2; // c == (2.5, 5)
c = b*2; // c == (12, 20)
```
Comparison for a typical use-case:
before:
```c++
CCPoint res = ccpAdd(ccpMult(ccpAdd(point1, point2), 2), ccpMult(ccpAdd(point2, point3), 2));
```
after:
```c++
CCPoint res = (point1 + point2) * 2 + (point2 + point3) * 2;
```