Updated the matrix exponentiation approach of finding nth fibonacci.

- Removed some extra spaces    
- Added the complexity of bruteforce algorithm  
- Removed unused function called zerro()  
- Added some docktest based on request

Author: mahbubcseju

matrix/nth_fibonacci_using_matrix_exponentiation.py

@@ -38,7 +38,7 @@ def identity(n):
 def nth_fibonacci(n):
     """
     >>> nth_fibonacci(100)
-    354224848179261915075L
+    354224848179261915075
     >>> nth_fibonacci(-100)
     -100
     """