Support for 2to3

Wireshark and Python are two of my favorite things, so I was pleased to find that pyshark lets me use them together.

I wanted to use pyshark with Python 3, so I looked into what I might need to change to port it. I think the changes needed are minimal - by making a few modifications I was able to get the library working without any version-specific code; 2to3 during setup can do the translation.

The changes I did make are:
* Call 2to3 `setup.py`
* Change a call to `file()` to `open()`, since the former is [deprecated](http://docs.python.org/2/library/functions.html#file) and the latter works on both Python 2 and 3.
* Make `Capture.next()` a proxy for a new method, `Capture.next_packet()` to smooth over calls from `super(FileCapture, self).next()` - 2to3 "fixes" that idiom incorrectly.
* Change some bare strings to bytes to allow both Python 2 and 3 to work with input data.

I've done a little testing and found I can read live captures and file captures on both Python 2.7 and 3.4 with these patches applied.

I'd be pleased to contribute to the project, and saw some other things I might submit patches for if you're good with these changes.