Started to implement TrPlaneStress2dXFEM, a three node triangle with
XFEM capability.
Added TrPlaneStress2dXFEMClass in classtype.h
Made some functions in XfemElementInterface virtual.
Added possibility to include headers from the Boost library and
continued with xfem implementation. Also implemented piecewise linear
polygon description of cracks and added routines for writing cracks and
cut xfem elements to vtk.
Added debug output for xfem.
Merge branch 'master' of git://github.com/Micket/oofem
Continued xfem implementation for cracks:
 * Heaviside enrichment can now be used with 3-node triangles
(trplanstrssxfem). (No crack tip enrichment available yet).
   NOTE: Boost must be included to simulate cracks.
 * Added small validation case for crack simulation: xFemCrackVal
 * Added combined implicit/explicit crack description with level sets.
 * Added some debug output to vtk for xfem.
Removed StVenantKirchhoff material since the same result can be obtained
by using linear elasticity with geometric nonlinearity.
Removed StVenantKirchhoffMaterialStatusClass
Removed stvenantkirchhoffortho.C
Merge branch 'master' of git://github.com/Micket/oofem
Corrected syntax in xfem vtkxml debug output.