Implementing time derivatives for auxiliary variables (closes #442)

- The implementation adds another instance of TimeIntegrator class onto
  AuxiliarySystem.  So the code we already have to compute our time
  derivatives does not have to repeated.
- It removes coupled_dot_aux_var test, which was testing the case when
  somebody wanted to coupled u_dot of an aux-variable. We do not need
  the test since we added that capability.
- It fixes DotCouplingKernel which was missing a test function, but was
  used only in the removed test and thus it was not an real issue.
- It improves the documentation of TimeIntegrators and brings the
  references to the current, old and older solutions into the
  sub-system, thus resulting in a nicer code.
- It adds a new test to verify the functionality of newly added
  capability.

Known limitations:
- u_dot of an auxiliary variable cannot be used inside the auxiliary
  system, because in order to compute the time derivative, we have to
  compute the new state of the aux. variable. Thus, this computation is
  done after the update has happened.  All systems after auxiliary
  system can bring these new values and use them with no problems.