Added a wrapper class for angles

`sf::Angle` works in a similar way to `sf::Time` providing conversion between radians and degrees in addition to basic mathematical operands.

Angles are always normalized to provide values within *[0, 360[ deg* resp. *[0, 2 PI[ rad*.

In addition, there's a constructor taking a floating point value to provide backward compatibility with existing source code.

Right now conversion offers radians as well as degree, although I could imagine adding [some more units](http://en.wikipedia.org/wiki/Angle#Units), such as *grad* or *seconds of arc*.