Added angle calculation methods to Vector2

Added two methods to the Vector2 class, allowing to calculate angle between two vectors, and not only between the x-axis.

I'm not using the standard acos solution, math explained here: http://geomalgorithms.com/vector_products.html

Since the dot product is the cosine of the angle between two vectors, and the 2d cross product (perp dot) is the sine of the same angle, plugging both on a `atan2(crs, dot)` call can give the angle on the full circle (-180~+180, instead of `acos(dot)` solution, which gives only 0~180) and also avoid the two sqrts involved in normalising the vectors.

Here's another explanation on the matter: http://johnblackburne.blogspot.com.br/2012/01/angle-between-two-vectors.html
Update Vector2.java

shouldn't have a conversion here
Added a StateMachine which is based on a Stack.
Bugfixes.
JavaDoc fixed.
Fix test particle file
Merge pull request #2189 from Tom-Ski/master

Fix test particle file
Exit only non-null current state.
Merge pull request #2181 from nooone/statemachine
Exit only non-null current state.
Update CHANGES
Merge pull request #2143 from Gtoknu/patch-8

Added angle calculation methods to Vector2
Fixed angle(Vector2) compilation.

#2143
Merge branch 'master' of https://github.com/libgdx/libgdx into enterstate
setInitialState uses changeState