Merge branch 'karlrupp/feature-refurbished-amg'

karlrupp/feature-refurbished-amg:
 Full rewrite of AMG functionality.
 Fine-grained AMG as described by Bell et al. now available.
 Includes GPU-accelerated setup.
 New implementation available for all three backends.
 Setup on host and application on device also possible.

Resolves #16.

Conflicts:
	viennacl/linalg/detail/amg/amg_coarse.hpp
	viennacl/linalg/detail/amg/amg_debug.hpp
	viennacl/linalg/detail/amg/amg_interpol.hpp