[NEW] added function for reading user input
Merge pull request #2 from shinypichu88/receive_string_input

[NEW] added function for reading user input
Create more Test Case
[Refactor] Move ExpressionTest Test Case to ExpressionTreeTest.java
[Add] more system test case
[Create] basic structure of Expression Tree
[Create] InputHandler to handle user input issue
[Infix2Postfix] convert from infix expression to postfix expression
[Pass] testParseToPostfix() Test Case
[Refactor] Extract TreeNode, Operator, Operand from inner Class to a individual class
fix testParseResult()
[Delete] non-OO code
[ADD] Interface for input output
Change the calculator to use input handler instead of using the tree directly
[New] basic function for building expression tree (not yet implemented)
Merge pull request #4 from shinypichu88/feature/InputHandler

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

Conflicts:
	src/cal/Calculator.java
	src/cal/ExpressionTree.java
First try on changing infix to postfix
[TODO] handling for ( )
Can change infix to postfix

postfix to tree
Correction on parseResult