Pep8 tests added, adders, subtractors added

* We generally add files and dont check its pep8 indentation and then somebody has to change it, which is cumbersome. So adding a test will be good.

To make the code pep8 comapatible:

```` 
sudo pip install --upgrade autopep8
````
For autopep8ing a file 
````
autopep8 --in-place --aggressive --aggressive <filename>
````
For autopep8ing a folder 
````
autopep8 --in-place --recursive --aggressive --aggressive <foldername>
````
After that, to check the pep8 errors

````
pep8 --ignore=E501 ./
````
* Binary Adder, Half subtracter, full subtracter, binary subtracter added.
* error due to __repr__in gates corrected
```` 
>>> OR(AND(0, 1), 0)
1
````