Create CrackDataSampler

closes #4377
This is a new VectorPostprocessor that outputs fracture domain integrals for points
along a crack front, with their angular positions used for the "id" field.
I agree that it would be better to do that, and I considered that.  In the interest of being consistent with other places in solid_mechanics where we use 0, 1, and 2 for the X, Y, and Z axes, I decided to keep it as is in this commit, but I'd like to make an across the board change to an enum.