Add custom net.spy.memcached.auth.AuthDescriptor.
Fixes issue #6.

+ Fix SASL authentication errors for Heroku users.
+ Allow passing an optional auth-type argument to
  spyglass.client/bin-connection.
+ Pull out net.spy.memcached.auth.AuthDescriptor
  construction to spyglass.client/auth-descriptor
  function. Passing :plain or :cram-md5 keywords
  specifies an auth mechanism. (AuthDescriptors
  initialized with AuthDescriptor/typical threw
  authentication errors for users connecting to
  Heroku's default memcached servers).
Merge pull request #9 from ecmendenhall/custom-auth-descriptor

Add custom net.spy.memcached.auth.AuthDescriptor.