Add Context.__len__() method

`Mapping.__len__()` is abstract and `Context` wasn't implementing it.  Normally, `Mapping`'s metaclass would therefore prevent `Context` from being instantiated, but this check is not performed due to `Context`'s multiple inheritance:

    >>> from raven.context import Context
    >>> Context()
    <Context: {}>

However, when gunicorn is used with gevent workers, gevent supplies its own implementation of `threading.local` which does run the checks:

    >>> import gevent.monkey
    >>> gevent.monkey.patch_all()
    >>> from raven.context import Context
    >>> Context()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "[...]/gevent/local.py", line 141, in __new__
        self = object.__new__(cls)
    TypeError: Can't instantiate abstract class Context with abstract methods __len__

Fix by implementing the method.