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
Multiply method for numeric arrays.
Path: Small changes
Merge pull request #959 from xoppa/path

Path: add derivative
Fix previous commit
Sort TexturePacker2 input files to avoid atlas changes across platforms
Merge pull request #961 from batesj/master

Sort TexturePacker2 input files to avoid atlas changes across platforms
Add missing Comparator import to TexturePacker2
Merge pull request #962 from batesj/master

Add missing Comparator import to TexturePacker2
Minor optimization
Merge branch 'origin/master'
Added support for arbitrary XML comments
Merge pull request #964 from hneuer/xmlcomment

Added support for arbitrary XML comments
Default JSON and XML readers to UTF8.
Add UBJsonWriter