Changed EmptySymbolException to EmptyTokenException.
This just makes more semantic sense because it is thrown from the class
Token.
Partitioned the conversion portion of Converter into a new class
- Created Class InfixToPostfix.
	- Has two static methods, convert(String infix) : List<Token>,
	  and convertToString(String infix) : String
- Created InfixToPostfixTest.
	- Contains 8 tests that cover basic functionality of InfixToPostfix and
	  test both convert and convertToString.
- Updated README.md to reflect new more limited scope of project.
Finished Refactoring Converter into two pieces.
- Created class PostfixEvaluator, which has a single static method
	evaluate(List<Token> postfix) : String
- Created class PostfixEvaluatorTest, which contains basic test cases
	for PostfixEvaluator, based on the test cases in InfixToPostfix.
- Removed Converter and ConverterTest: Converter is now completely
	separated into PostfixEvaluator and InfixToPostfix.