Improved File.download

[tsnoam]

1. Added a new method File.download_as_bytearray() which returns a bytearray. Simple and straight forward for both py2 & py3.
2. File.download() now returns a meaningful return value:
   • If download was saved to a file, return the file name.
   • If download was saved into a file-file object, return the file-like object.

Fixes #944

[Eldinnie]

I pushed some small fixes for the docstrings.
The rest seems good to me.