Pytube

* Improved support for additional channel URLs
* Improved metadata for channels
* Improved channel documentation
* Change to `get_video_info` call to fix 404 issue

* Fixes an issue where playlist lengths were not being cast to ints correctly for large playlists.

* Implements an actual check for whether a video is a livestream, and raises an exception if it is.
* Fixes channel url matching for channels without canonical names.

* Fixes the 404 issue by adding html5=1 as an additional query parameter to the get_video_info endpoint

* Prepends the URL used with www. to avoid the redirect that occurs.

* Added channel id and channel url properties to YouTube object.

* Added some metadata to playlist object:
- owner
- owner_id
- owner_url
- description
- length
- views

* Removes file write chunk size to improve performance.

* Implements a Channel object for downloading videos from a YouTube channel.

* Minor changes to the playlist class to make it more compatible to be subclassed.

* `.videos` and `.video_urls` now behave just like iterable lists, but defer web requests.

* Implements DeferredGeneratorList which converts generators to lazy list-like objects.