Add new method for deleting multiple members from a list and made version number in the url configurable

I wanted to use the method described under url https://dev.twitter.com/docs/api/1.1/post/lists/members/destroy_all. This method was not present in twitter_endpoints.py. Also the version number in the file twython was coded to '1'. Now, I moved this into the __init__ method so that folks can configure the url version when they construct twython instance. 'version' defaults to '1'. I tested this method using my own oauth keys. I hope this is useful to other users.