Version 0.5.3

This release enables using custom gems in the leap platform
customization. It also fixes cornercases during the account creation and
documents debugging in production.

 * android app now supports signup, so change text that said otherwise.
 * added debugging note to DEVELOP.md
 * Account.create - do a User.new instead of User.create, so that we can
   report the errors on the object if not saved.

Pull request #172 from elijh/feature/customgem
 * Gemfile: fix problem when config is missing environments
 * support for optional gems in Gemfile (engines/ and
 * config/customization/gems/)

Pull request #171 from elijh/feature/identityfail
 * if identity fails to be created, destroy the user. also, pass through
   identity errors to user and add identity class hook.