Faster matrix power

Hello.

I noticed that in math.js power of matrix (A^P, size(A)=N) code has computational complexity O(N^3P) that can be improved to O(N^3logP). Optimized code is included in my pull request. I also added some test cases in pow.test.js:

- power 0 of square matrix (A^0) test
 - in original code it fails. 

- power of larger size of square matrix test
 - in original code it takes long (in my environment >= 10s) time.

I'd appreciate it if you could give me some feedback. Thanks.