Bring Gist object into line with module style.
Bring Gist object into line with module style.

On other objects, attributes like 'comments' usually indicate the number of
comments on that object where as methods like 'list_comments' actually return
the comments. Here, 'files' was an attribute returning a list of GistFiles
instead of the number of files. So I thought I should change that attribute to
the number of files on the Gist and add list_files() to return the list of
files.

agamdua/github3.py,icio/github3.py,h4ck3rm1k3/github3.py,balloob/github3.py,ueg1990/github3.py,wbrefvem/github3.py,christophelec/github3.py,sigmavirus24/github3.py,krxsky/github3.py,jim-minter/github3.py,degustaf/github3.py,itsmemattchung/github3.py