Add custom naming to decorator

[MasonEgger]

It would be nice if we could specify an optional metric name through the decorators instead of getting <FUNCTION_NAME>_calls.

Something like

    >>> from pyformance import counter, count_calls
    >>> @count_calls(metric_name="test.count")
    ... def test():
    ...     pass
    ...
    >>> for i in range(10):
    ...     test()
    ...
    >>> print counter("test.count").get_count()
    10