wrap the current functionality into a mode

in the future, the mode can be injected

discourse/discourse-crowd#3
add mixed mode of crowd authentication

When this mode is enabled, crowd authentication works even for accounts,
that were created not during initial login by crowd.

discourse/discourse-crowd#3
update readme with needed setting of crowd mode

resolves discourse/discourse-crowd#3
create a class for each of the crowd login mode

discourse/discourse-crowd#3
preserve backwards compatibility

The `separated` mode is default and will work even if the
`crowd_login_mode` settings is not defined.

discourse/discourse-crowd#3
Merge pull request #4 from coubeatczech/i3

I3