Devise installed, SASS structure

Installed devise following the instructions on the repo. Letter opener
has been configured for development. Home controller added with Devise
authentication on User model, primarily for testing. Site will be open
by default.

Devise views have been converted to Slim and SASS structure is in place
so that Pure can be extended with ease. Possibly add Compass/Bourbon but
there doesn't seem to be much need right now.