Wff: Change return type of parse function

Change 'parse :: String -> Maybe Wff' to 'parse :: String ->
Either String Wff' in order to provide error messages. Also
change 'parseAll :: [String] -> Maybe [Wff]' to 'parseAll ::
[String] -> Either String [Wff]'.
Proof: Refator Wff module.

Refactor 'isAxiom :: Wff -> Bool', 'isModusPonens :: Wff -> Wff -> Wff
-> Bool', 'isSubstitution  :: Wff -> Wff -> Bool', and 'isProof :: [Wff]
-> [Wff] -> Bool' from Wff module to a new Proof module.
TestWff: Rename TestTheorem module to TestWff
TestTheorem: Run all tests in TestTheorem.hs.
TestWff: Add testParseAllError to allTests
.gitignore: Ignore Notepad++ session file
Theorem: Format tests