Add Support for Seeding Identities

This pull request is for issue [RavenDB-1206](http://issues.hibernatingrhinos.com/issue/RavenDB-1206) which allows seeding of identities in the database.

The server endpoint is `/idientiy/seed` which will accept a `POST` method and `name` and `value` as the query string parameters.

The client has a method called `SeedIdentityFor` in the database commands interface.  The expected output of the method is the value that was sent to the server.