Adders, Subtractors added , IC4027, 4028, 4029 added, Examples with all new functions.

* Binary Adder, Half subtracter, full subtracter, binary subtracter added.
* error due to __repr__in gates corrected
```` 
>>> OR(AND(0, 1), 0)
1
````
* Changed ```` backtrack```` to ````Tree```` class, so that we can have forward traversing also and can show tree for forward traversal, tests changed, Examples added
* Changed 2-bit Binary Counter to n-bit counter, so that it can also serve the purpose of UP counter
* Extra functions in TFlipFlop removed which were creating problem due to overriden functions from JKFlipFlop
* Corrected parseEquation.py, Added its tests and examples.
````
Expr('(A & B)').parse()
Invalid Arguments
````
*  Examples added for new classes, Adders, Subtractors, Octal counter, stage14counter, connector, expr etc.
* Examples for Combinational devices and gates corrected, added __repr__ and __call__ type examples
* Refactored examples for counters, and sequential circuits, added examples of all its varied functions like ````setInputs````, ````setOutputs````, ````setCounter````, ````resetCounter```` etc.
* Added IC 4027, 4028, 4029 related to counters and sequential circuits, with tests.
* Added tests which were not added in ````.travis.yml```` , ````print_tests````, ````backtrack_tests```` etc.

Now almost all of the features of BinPy has examples.