Matrix Inverse Implementation

A function that computes the inverse of the special "matrix", which is
required by https://class.coursera.org/rprog-005
Revert "Matrix Inverse Implementation"

This reverts commit 2b9dfa277a2e9dd1c866caea8ebd3979dd546348.
Revert "Revert "Matrix Inverse Implementation""

This reverts commit 58725aa053fe9daacd557bb711d08c7d97546ed8.
Add Comments

Add comments to both two functions