Add a parser for IDPSSODescriptor a.k.a. IDP metadata

[fumieval]

I implemented a parser for IdP metadata. There are a few remaining design questions; I'm open to suggestions.

• Should it parse the contents of x509Certificate?
• Should there be a datatype for NameIDFormat? If it should, It would affect the definition of NameID too

[fumieval]

I updated the type of x509Certificate to SignedExact Certificate and added some documentation. Unfortunately I wasn't able to find specification for EntityDescriptor in HTML, so I linked to the PDF version instead

[fumieval]

I'll implement a datatype for NameIDFormat in a separate PR

[fumieval]

I updated the code style a bit. It would be nice if you could document non-standard code style requirements (such as -- | haddock syntax for record fields) in the guideline

[mbg]

It would be nice if you could document non-standard code style requirements

No problem, if this is helpful I can add it explicitly to the contribution guidelines.

[mbg]

@fumieval I have just opened #25 with some improvements (hopefully!) to the contribution guidelines. I would be grateful if you could have a look over them to see if that addresses everything you'd find useful or if anything is missing?

[fumieval]

Rebased to use the NameIDFormat datatype