[Create] InputHandler to handle user input issue
[ADD] Interface for input output
Change the calculator to use input handler instead of using the tree directly
Merge pull request #4 from shinypichu88/feature/InputHandler

Feature/input handler
Fix bug for invalid input format
Merge branch 'develop' of https://github.com/shinypichu88/CS3343-Cal into feature/ExpressionTree

Conflicts:
	src/cal/Calculator.java
	src/cal/ExpressionTree.java