add ftps adapter, based on double_bag_ftps

Not much to say about this, I needed to some special parameters for a certain deployment project, and fortunately this was pretty easy to accomplish since double-bag-ftps was just an extension on top of Net::FTP.  So I just leapfrogged your original Dandelion::Adapter::FTP and replaced the ftp object it constructs.  Also added a few config options to support it.  Kudos for a building such a clean structure that facilitated this. 

These config options got me to where i needed to be for this particular ftp server.  but there may be more options  required or better ways of articulating connection parameters (auth_tls, for example, maybe allowing user to pass in an arbitrary string would be better).  i'm not really completely familiar with all the nuances of ftp encryption.

And, I hate to say, I'm not really familiar with ruby either, this is really my first foray.  I only feel comfortable publishing this because the task was fairly trivial, and maybe it will be useful to others