Support for nonlocal declaration

This PR implements #494.

There is one note I should make: An error message is generated when `nonlocal x` follows a definition of `x`, like in this case:

```python
def outer():
    x = 1
    def inner():
        x = 2
        nonlocal x
    inner()
    print(x)
```

However, Python 3.4 reports just a warning. I made this choice, because it was the easiest way to report an error (as it should) when there is a `nonlocal` declaration of a variable with the same name as a function parameter:

```python
def outer():
    x = 1
    def inner(x):
        nonlocal x
```

I don't think it is such a bad idea to report an error in the first case too. In the first code snippet, the output is `2`. Which is (at least to me) a bit counter intuitive as the `nonlocal x` apparently affects an earlier statement. Furthermore, generating an error message in this case does not exclude any uses of `nonlocal` as it can be moved to the front of the function with changing the semantics.

Nevertheless, if you feel this is overly restrictive, let me know and I will try to find another way of dealing with this.
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