Add some tests for LCS TimeEfficientImplementation.
Improve TimeEfficientImplementation performances.

Using a SplFixedArray seems to reduce memory allocation (-90% memory peak usage) and have greater lookup speed (+15%) than the two-dimension dynamic array storing the LCS matrix. It also seems that a SplFixedArray is wiped earlier by the PHP garbage collector.

An other optimization: avoid usage of "array_unshift" (Pushing into the result array and reversing it at the end is faster in most cases).

If this patch is OK, the "Differ->calculateEstimatedFootprint()" should be also adjusted, because the SplFixedArray memory cost seems to be completely different than the one of (partially filled) arrayXarray matrix.

(+ Removed unclear comment in the TimeEfficientImplementationTest case.)
Merge pull request #22 from gcrico/Improve-LCS-TimeEfficientImplementation

Improve LCS TimeEfficientImplementation