Use git:// urls for deps
Move to using crypto:hash instead of deprecated crypto:sha
Move binding-specific logic into its own module

This is preparation for splitting up the cowboy_handler
module further.
Break up esaml_cowboy_handler and remove the esaml_sp behaviour
Add records and xml for single logout
Add generate_ functions to esaml_sp for SLO
esaml_sp validators for SLO
Add IDP metadata fetch, more SLO stuff