Add support for the power operator **

Also renamed UnitTest1 to Parseable and did some refactoring
Write some tests for infix notation

Rename Input() method to PostfixInput()
Implement the Shunting-yard algorithm for infix input
Reorder arguments for tests
Use ^ instead of ** as exponent operator
Support for parentheses ( and ) in infix notation
Use Dictionary instead of array
Add some helper methods for improved readability
Handle parentheses and left/right-associativity correctly
Run cleanup
Answers to exercises for OOSE