Minimal call sequence verification implementation

This is the best API I could come up so far. An example of proposed syntax would be something like this:

    var sequence = new CallSequence();
    var mock1 = new Mock<RoleWithSingleSimplestMethod> { CallSequence = sequence };
    var mock2 = new Mock<RoleWithSingleSimplestMethod> { CallSequence = sequence };
    var mock3 = new Mock<RoleWithSingleSimplestMethod>();
    
    mock1.Object.Do();
    mock3.Object.Do();
    mock2.Object.Do();
    
    CallSequence.Verify(
      mock1.CallTo(a => a.Do()),
      mock2.CallTo(b => b.Do())
    );

Regarding discussion in issue #21 :

1. Removed "verify not in sequence" feature for now
2. Changes CallSequence to take parameter of MockBehavior

        var m1 = new Mock<RoleWithSingleSimplestMethod>() 
            { CallSequence = new CallSequence(MockBehavior.Loose) };
    

3. Introduced a shortcut in CallSequence when all mocks have the same sequence:

        CallSequence.Verify(
          m1.CallTo(m => m.Do()),
          m2.CallTo(m => m.Do()))
        );

  A method is exposed on sequence object should anyone require checking more than one sequence:

        sequence.VerifyCalls(
          m1.CallTo(m => m.Do()),
          m2.CallTo(m => m.Do()))
        );

4. Moved the sequencing feature (along with `CallTo...` extension methods) to a separate namespace
5. Introduced extension  points but for now they are hidden because I have no good idea yet on how to deal with Times.XYZ() - for sequences, this is non-trivial, especially when taking into account few combined "at least" and "at most" checks.
6.Exposed CallSequence on MockRepository
no underscores in fields plz ;) (if you don't provide an auto-prop for this, see next comment