Add support for multiple Assertion Consumer Services (ACS)

[mauromol]

This is an enhancement request to make java-saml support multiple Assertion Consumer Services.
This means:

• the ability to configure multiple ACSes (URL and binding, with index and the indication of which is the default one) in the settings, with corresponding metadata rendering
• the ability to select which ACS the response should be targeted to when creating an AuthnRequest

I can provide a PR for this. Here is a preview: mauromol@54a1459
It's not ready for opening a PR though, because the work is based on previous work (mainly #307 and #315).