Magic versions

When I try to add a signature using alot, I get the following error

    DEBUG:globals:has signature
    DEBUG:globals:is file
    ERROR:ui:Traceback (most recent call last):
      File "/home/hamish/.pythonbrew/pythons/Python-2.7.3/lib/python2.7/site-packages/twisted/internet/defer.py", line     576, in _runCallbacks
        current.result = callback(current.result, *args, **kw)
      File "/home/hamish/.pythonbrew/pythons/Python-2.7.3/lib/python2.7/site-packages/alot/ui.py", line 609, in call_apply
        return defer.maybeDeferred(cmd.apply, self)
      File "/home/hamish/.pythonbrew/pythons/Python-2.7.3/lib/python2.7/site-packages/twisted/internet/defer.py", line     138, in maybeDeferred
        result = f(*args, **kw)
      File "/home/hamish/.pythonbrew/pythons/Python-2.7.3/lib/python2.7/site-packages/twisted/internet/defer.py", line     1214, in unwindGenerator
        return _inlineCallbacks(None, gen, Deferred())
    --- <exception caught here> ---
      File "/home/hamish/.pythonbrew/pythons/Python-2.7.3/lib/python2.7/site-packages/twisted/internet/defer.py", line     1071, in _inlineCallbacks
        result = g.send(result)
      File "/home/hamish/.pythonbrew/pythons/Python-2.7.3/lib/python2.7/site-packages/alot/commands/globals.py", line 726, in apply
        enc = helper.guess_encoding(sigcontent)
      File "/home/hamish/.pythonbrew/pythons/Python-2.7.3/lib/python2.7/site-packages/alot/helper.py", line 396, in        guess_encoding
        m = magic.open(magic.MAGIC_MIME_ENCODING)
    exceptions.AttributeError: 'module' object has no attribute 'open'

Two answers on [this stackoverflow question](http://stackoverflow.com/questions/43580/how-to-find-the-mime-type-of-a-file-in-python) suggest that the API had changed by 2010. The current Ubuntu release package installed via apt still has the older version though (with `magic.open`) 

I have installed python-magic using pip, and it doesn't seem to want to offer me anything older than 0.4.3  (Unless I have  installed entirely the wrong package from pip?  I have spent a while looking at various magic file python libraries and haven't found a better one yet.)

This patch checks if magic has the open attribute, and if not it will use the newer version of the magic bindings.