Add functionality to chain patch operations on multiple fields

[mysticmind]

Refer to GH-699 for details. You can perform patch operations as below:

theSession.Patch<Target>(target.Id)
    .Set(x => x.Number, 10)
    .Increment(x => x.Inner.Number, 10);

[jeremydmiller]

Looks good to me! I'm pulling this in.