Use explicit init.comm in examples

This should let "make check" pass with --disable-default-comm
eventually.  For now, we break when we get to the adaptivity examples
because there's nothing with a Communicator initializing the
ErrorEstimator subclasses.  I think I want to fix that one with some
compatibility work on ErrorEstimator, though, not by requiring more
changes to user code.
examples fix for --disable-default-comm-world
Fix another CommWorld-using example
Fix accidental omission of fem_system_ex2 Mesh dim
Example SumShellMatrix creation should use a comm

This fixes miscellaneous_ex4 in the --disable-default-comm-world case
Add communicator for miscellaneous_ex6 SerialMesh
miscellaneous_ex7 --disable-default-comm-world fix
reduced_basis --disable-default-comm-world fixes
Add and use LibMeshInit::communicator() accessor
Merge pull request #79 from roystgnr/examples_fixes

Use explicit init.communicator() in examples
Optionally disable default CommWorld args

The library doesn't yet compile with all new default Communicator
arguments disabled, because some of those arguments follow other
arguments with defaults.  We'll need to add some overload shims to fix
that.
Add LIBMESH_CAN_DEFAULT_TO_COMMWORLD macro

And reorder new method args as necessary to support it.

Because I just redid our examples to use the new methods on Mesh
classes and because that was one of the necessary reorderings, this
won't pass "make check" yet; a subsequent commit should fix that.