Added ViennaCL backend

amgcl::backend::viennacl< viennacl::hyb_matrix<double> >
will work when viennacl/viennacl-dev#79 is merged.
Removed CCRS backend

It does not make sense unless setup uses compression as well
Added classic Ruge-Stuben coarsening
Smoothed aggregation with energy minimization
Reorder iterative solvers parameters

This also adds shortcut for the case when the system matrix coincides
with the one used for the construction of AMG.
Relaxation scheme is a class template intead of enum

This makes possible adding new smoothers without changing core sources
Added ilu0 smoother
Renamed spai.hpp and ilu.hpp to spai0.hpp and ilu0.hpp
Reorder code in backend/vexcl.hpp for clarity
Added README
Added LICENSE
Added Doxygen configuration
[WIP] Documentation