Fix javascript 'float' and 'int' functions

float("14") < float("2")    now returns 'false'
float('a') int('a')         raises an exception

window object is no more used in order to be compatible
with server side JavaScript
Javascript String.index may throw an exception as in Python. Add String.find

String.find is the same than the former String.index
Fix the translator in the case of Python code outside of functions

The following code is now translatable:

a = {'x': '5'}
print a['x']
Rewrite of the 'dict' getters and setters

With this rewrite:
  * The getter raise an exception if the item does not exist
  * '@' prefix is replaced by '￼' because it is less common
  * 'uid' attribute has been replaced by '__uid__'
  * '__contains__' now works with keys that are functions

The code runs at the same speed.

As before, 4 and '4' are the same key.
Add missing exception values: IndexError, KeyError, ValueError

Put the good exception names in the buitins.
Merge pull request #60 from texcoffier/master

Fix javascript 'float' and 'int' functions, and fixes to exceptions.