Add config option for specifying SignatureMethod/DigestMethod

[joetsoi]

It would be nice if you were able to specify a default signing or digest method, instead of having to specify it as a parameter to Entity/Saml2Client.sign() and just be able to specify in an sp's config which signing algorithm it is using.

{
    'service': {
        'sp': {
        'signing_algorithm':  saml2.xmldsig.SIG_RSA_SHA256,
        'name': 'blah',
        ...
       },
   }
...
}

Is there a better method of doing this already in the config that I've missed?

Adds signing_algorithm and digest_algorithm to Entity.
If sign_alg or digest_alg is not provided as a parameter to
Saml2Client.sign(), then check if one has been provided in the config
before falling back to sig_default in xmldsig.

[c00kiemon5ter]

continue this on #597