Use explicit init.comm in examples

This perhaps shouldn't be pulled as-is, but Derek *just* correctly reminded me not to let stuff linger too long out of trunk, so I should get this up for public perusal.

I want our "make check" to compile and run with --disable-default-comm-world.  This patch doesn't yet completely do that, because of the ErrorEstimator issue, but it's an improvement.  What it mostly needs before committing is some consensus on how to get a communicator into that first Mesh for everything else to use.  I'm currently passing in LibMeshInit::comm but my current thoughts are that it would be better to future-proof that interface by making comm private and adding a communicator() accessor.  Other opinions?