Fixing bug in login template when registration is disabled.

There was a bug that occured when a user had STORMPATH_ENABLE_REGISTRATION
disabled -- the login page would error out since it was linking to a
non-existant view.
Fixing bug which occurs when login is disabled.

If a user had STORMPATH_ENABLE_LOGIN disabled, and tried to visit the
registration view (/register), it would error out because the registration page
would link to the login page.

This is now fixed!
Version bump!
Adding changelog docs.
Adding upgrading docs.