Move Plug and Phoenix code into another lib.

[vic]

Quoting the README file: Guardian remains a functional system. It integrates with Plug, but can be used outside of it.. However if the app does not depend on Plug nor Phoenix, a warning is emitted upon Guardian compilation:

=> guardian
warning: function Phoenix.Socket.assign/3 is undefined (module Phoenix.Socket is not available)
Found at 3 locations:
  lib/guardian/phoenix/socket.ex:71
  lib/guardian/phoenix/socket.ex:79
  lib/guardian/phoenix/socket.ex:87

One solution would be to move the Plug and Phoenix specific code from Guardian into another package, say guardian_plug. However, as I guess most people are using Guardian from inside a Phoenix app, I'm not sure if this is the right way to go. Also, moving the code out would require a major version change.

[doomspork]

@vic while I agree a warning is not pretty but I'm not sure if it warrants a wholly separate library. @hassox do you have any thoughts?

[vic]

Yep I know, just was used to having nicely clean builds, but now with elixir 1.4 almost any dep I have has some warning (regarding the ()) hopefully some day I'll have my eyes clean of warnings. :D cheers !