Added metropolis criterion.

Hi,

I never really used GitHub so I hope this is the right way to commit.

As you can see I tried implementing the metropolis criterion.
As mentioned in the assignment I used kT = 1.
Please review if I made any mistakes. 

If the implementation is correct, the next step would be to dump a pdb file and then answer the questions in part 2.
I will continue to work as soon as I can (sometime tonight)