Allow passing a proc for ldap_config

This allows providing a proc that returns a configuration hash to the ldap_config attribute.  This makes it possible to store LDAP configuration in a database or some other mechanism.

Existing functionality (loading from `ldap.yml`) remains unchanged.

For example:

```ruby
Devise.setup do |config|
  # Load configuration from database
  config.ldap_config = Proc.new() { MyCoolORM::Config.ldap_config }
end
```