Add derivative calculations to CatmullRomSpline

I added the calculation of the derivative of the spline. This finds the tangent of the curve at the desired point.
Path interface now has the method derivativeAt()

Path interface now has the method derivativeAt() that calculates the tangent of the curve at the desired point.
Added derivatives, commentaries with expanded formula

Added derivative calculation to linear, quadratic and cubic curves.
Added commentary to explain the curve and derivative formulas
Added derivatives to b-spline
Added derivative test to PathTest.java

made sprite to always point to the path direction, added a mode which the sprite will be zig-zagging through the path.
Merge branch 'patch-1' of http://github.com/Gtoknu/libgdx into path
Path: Small changes
Merge pull request #959 from xoppa/path

Path: add derivative