Updates to OpenIdConnectAuth

[clintonb]

• Added support for passing kwargs to jwt.decode()
• Updated iat claim validation

The first commit adds support for passing kwargs to jwt.decode(), allowing for great flexibility to determine, for example, which signing algorithms are used or the expected value of the audience claim.

The second commit (a) instructs PyJWT to verify the iat claim and ensure a value is present, and (b) makes the value against which the claim is compared configurable. I don't know why I hardcoded this to 10 minutes; but, that is not in the spec, and should be configurable for each individual client.

[clintonb]

@omab please reivew

[omab]

Merged, thanks!