Tidying of code

Added some regular expressions.
Tidying of code

Added some regular expressions.
Preprocessing updated.

Re-wrote the 'check_paren' functien which is now much easier. The parse function raises the right exception if
the check_paren function sees that they parentheses do not match. Also, whitespace is stripped. 11 tests passed.
Added special charcters to the symbols
Added the handling of 'quotes'