Added $parameters array to OneLogin_Saml2_Auth->login()

I'm currently using OneLogin_Saml2_Auth to authenticate with an identity provider that requires additional parameters to be sent through along with the SAML request. The login method doesn't currently have the option to pass through additional parameters to the redirection methods, so I've added a $parameters array to the method in order to allow you to pass through additional parameters.

Also, I've added a bit of code to pull them out of the idp section of the settings if it's defined there.