Correct aux exec machinery 2172

This patch repairs two big issues in MOOSE and provides legacy flags to maintain existing behavior while we update and fixup the herd:

1. (#2172) We compute the EXEC_RESDIUAL aux kernels whenever we compute *any* UOs.  See FEProblem.C:2205.  We should be executing matching types (i.e. EXEC_TIMESTEP <=> EXEC_TIMESTEP, etc.)

2. We compute all UOs during initialSetup() regardless of type. This includes Postprocessors.  While this doesn't sound too bad, the new execute_on system allows finer control over this behavior.  Users that have Postprocessors that need to run during the simulation but should also run up front can now accomplish that with `execute_on = 'initial timestep'` or something similar.  

Both of these changes can be controlled at the application level by making an explicit call through main.C or on a test by test basis through an input file parameter.  For now, all applications are defaulting to "true" for both of these legacy flags, meaning that we can fix the applications one by one.