get_highest_res() method addition

Submitting a proposal for a new method called get_highest_res(). It allows the user to automatically get the highest Youtube resolution. They can also specify the preferred file format, though it will favour highest quality over preferred format.

Here it is in action against your Readme example:

```
>>> from pytube import YouTube
>>> from pprint import pprint
>>> yt = YouTube()
>>> yt.url = "http://www.youtube.com/watch?v=Ik-RsDGPI5Y"
>>> pprint(yt.videos)
[<Video: MPEG-4 Visual (.3gp) - 144p>,
 <Video: MPEG-4 Visual (.3gp) - 240p>,
 <Video: Sorenson H.263 (.flv) - 240p>,
 <Video: H.264 (.flv) - 360p>,
 <Video: H.264 (.flv) - 480p>,
 <Video: H.264 (.mp4) - 360p>,
 <Video: H.264 (.mp4) - 720p>,
 <Video: VP8 (.webm) - 360p>,
 <Video: VP8 (.webm) - 480p>,
 <Video: VP8 (.webm) - 720p>]
>>> yt.get_highest_res('webm')
<Video: VP8 (.webm) - 720p>
>>> yt.get_highest_res()
<Video: H.264 (.mp4) - 720p>
>>> yt.get_highest_res('flv')
<Video: H.264 (.mp4) - 720p>
```