Add support for MDUI attributes in any endpoint, IdP or SP

MDUI attributes were being used only on SP endpoints, but they can
happen in IdPs too.

I've changed the order of the plugins, enforcing the *endpoints* plugin to execute first. The *info* plugin now relies on the existing `endpoint` en the `entitydescriptor` object instead of writing the information always inside a `saml2sp` object. Therefore, the endpoint must be created before executing the *info* plugin.

This pull request has been motivated by https://github.com/Yaco-Sistemas/peer/issues/18