Implemented the backtrack function along with some __str__() and __repr__() functions to have a more interesting terminal output.
Made changes so the code runs with Python 3
Merge pull request #156 from petermlm/develop

Outputs
Added multiplexer IC74152, IC74151A ,IC74153
Added tests 

for IC74152, IC74151A ,IC74153,IC74156, IC74155, IC74139 ,IC74138
Added demux ICs 74138 ,74139 ,74155 ,74156
Create IC74156.py
Create IC74138.py
Create IC74139.py
Create IC74151A
Create IC74153.py
Rename IC74151A to IC74151A.py
Create IC74152.py
Imported combinational.py
Merge pull request #162 from mohitgujarathi14/combIC

Added Multliplexers and Demultiplexers
Modified the int method of connector to avoid None as an output
Added methods in connector class to check inp / outp relationship
Added untap method in connector class
PEP8
Commented out print lines in backtrack tests