Access token branch saif

FIles To Review

1..AccessTokenDataBase.cs
2..CustomException.cs
3.IAccessToken.cs
4.TokenAccessService.svc
5.AccessTokenFixture.cs

Couple of points here - 
1. For the use case of storing and retrieving tokens, the list data structure is a poor choice. Given that you are going to make unordered key based access with add, remove and contains operations, a much better choice would be [HashSet\<T\>](http://blogs.msdn.com/b/bclteam/archive/2006/11/09/introducing-hashset-t-kim-hamilton.aspx).
2. Class names should always be explicit. A better name would be something like `HashSetBasedAccessTokenDb` or `InMemoryAccessTokenDb` etc.
3. Which is a better option as per you and why?
      -   Keeping a private static list.
      -   Keeping a per instance list but keeping the whole token database instance static.
This is more for discussion than for any specific guidance. Any thoughts ??