Work with Azure AD

[srenatus]

Hi there.

Trying to get esaml (branch develop) to work with Azure, the following things came up:

1. the c14n issue already fixed by @zuckschwerdt in fix c14n canonization #15,
2. the Azure IdP rejects Authn Requests that include a Subject (although the docs claim that it'd be ignored)
3. by default, Azure's AD doesn't respond with NameId of format urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress

There's a combined commit for 2. and 3. here, but this solution might not be the only possible one. What do you think is the best way for azure compatibility? Just remove the Subject (2.) and let the user cope with that NameID comes in the response, or expose the NameIdPolicy to make it configurable (in esaml_sp)?

Thanks