Support for federated authentication / enterprise SSO - take 2

started from scratch because the other PR (#525) was too cluttered already after so much back and forth and merge/rebase.

* It uses [Auth10.AspNet.SimpleConfig](https://nuget.org/packages/Auth10.AspNet.SimpleConfig) nuget package that provides easier WIF integration using appSettings instead of microsoft.IdentityModel
* depends on [Microsoft.IdentityModel](https://nuget.org/packages/Microsoft.IdentityModel) nuget (which is just the WIF dll that has proper license to redist now)
* Support Windows Azure ACS and any other WS-Fed + SAML token idp
* Provides a selector like janrain that uses Windows Azure AD/ACS. This is optional (selectorEnabled = true/false in config)
* Logout will cleanup fedauth cookie
* Logic for linking identity of the federated user (being janrain or wif) moved to its own service IdentityLinker
* The feature can be turned on/off by entering empty values on the fedauth.* appsettings

**What scenarios this enable?**

* **Enterprise chat with SSO**: Organization that wants to deploy Jabbr on the cloud or on-premises but have their employees only access with Active Directory (using ADFS or any other identity provider)
* **Chat multi-enterprise with SSO**: Multiple organizations that wants to deploy Jabbr and have a private chat system between them. They could rely on Windows Azure ACS and allow users logging in with different identity providers (even social idps like google, facebook, etc for those who don't have ADFS)
* **Chat for Office 365 orgs**: Small/mid business with a subscription with Office 365 and wants to have a private chat system for their employees and single sign on using Office 365 credentials