Updated exceptions to be more fitting, fixed a parse bug in the tokenizer and added the start of the calc engine.

main.py

@@ -1,7 +1,7 @@
 import token
 import tokenizer
 #userInput = input('Enter your formula: ')
-tokens = tokenizer.get_tokens_from_expression_string("1 + 2 / 3 (3 * 4) 5")
+tokens = tokenizer.get_tokens_from_expression_string("i + 2 / i (3 * n) 5")
 
 for t in tokens:
 	print("%s (%s) " %(t.value, token.TokenType.get_operator_name(t.type)), end = "")