[Create] basic structure of Expression Tree
[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
[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
[Done] isExpression()
Revise isExpression(), input bug exist here
Make function for create postfix ArrayList
Input infix String: 1+(2.2+333)*444
Postfix ArrayList: [1, 2.2, 333, +, 444, *, +]
Clear up un-used code
Fix minor mistake
Add InputHandlerTest
pass 5 test case