Added MAJOR, MINOR, and REVISION constants.

This is useful for frameworks that want to be compatible with different versions of LibGdx.
For example if version 1.2.0 will fix an issue and you know a workaround for previous versions then in your code you can quickly do something like this 
````java
if (Version.MAJOR == 1 && Version.MINOR < 2) {
   // Apply workaround
}
````
No more stupid string comparison :)