Add MeshBase::get_boundary_info() accessors

This commit also removes a redundant public: declaration in MeshBase.
New API for const sideset/nodeset name lookup

We never caught the constness problem before because we were always
grabbing non-const BoundaryInfo objects from const MeshBase objects.
Store reference, not pointer, to BoundaryInfo
Use get_boundary_info() in library code
Use get_boundary_info() in examples

This also removes some unnecessary inclusions of boundary_info.h
Merge pull request #388 from libMesh/get_boundary_info

get_boundary_info accessors