Add Binary Tree implementation, tests

[rehassachdeva]

references #4

Please review.

[goelakash]

Good work! @rehassachdeva . Few comments:

1. delete is slightly trickier. You can only delete children node when they are leaf nodes, other wise you have to some adjustments. See this to understand more - http://www.geeksforgeeks.org/write-a-c-program-to-delete-a-tree/
   In case of two children nodes, you may choose to make any one of them as the new parent for the other node. Then follow that procedure recursively. You can use any traversal technique here.
2. Add docstrings to functions to explain the arguments and the operations being performed on them.

[rehassachdeva]

Hi @goelakash. Have made the necessary changes. Please check now

[goelakash]

@rehassachdeva Looks good to me. Can you please add some the docs as well?
@aktech Can we merge this?

[goelakash]

#74 Maybe we can have a separate traversal module, and use those for all tree data structures. @aktech @CuriousLearner What do you guys think?

[rehassachdeva]

@goelakash Docs have already been added here #68. I'll add reference for binary_tree once this is merged?

[goelakash]

@rehassachdeva That sounds good.

[aktech]

@rehassachdeva Please fix conflicts.

[rehassachdeva]

@aktech