Add VectorPostprocessors to get materials along a line closes #4462

This provides an abstract base class, LineMaterialSamplerBase, that
has the basic machinery for extracting the material properties for the
elements along a line, and two derived classes,
LineMaterialRealSampler and LineMaterialSymmTensorSampler, for
extracting data for specific material types.
Merge pull request #4463 from bwspenc/material_vectorpost

Add VectorPostprocessors to get materials along a line