use tuples instead of lists if the don't change (immutable is faster and more memory efficient)
fixed a problem with docstrings that were empty (None), docstrings in Jedi are always strings.
basic precedence algorithm - not tested yet
precedence tests and a parse_tree method for Precedence objects
precedence working for some simple cases (and invalid statements)
MultiPart string precedences seem to be working now
to the power of precedences
get the 'not' priorities right for precedences