Fix javascript 'float' and 'int' functions

Hello, I am a new user of PythonJS and GitHub.
So, forgive my errors...

I am not sure about a point in my commit on float/int : 
it does not raise an exception on float("") or int("") as in Python.

The commit about String.index may break regression tests,
but I don't know how to run them automaticaly.

The 'dict' commit is a full rewrite, I hope I did not make mistakes.

The 'exception' commit add missing declarations.