ViennaCL 1.3.1 released!

The following bugfixes and enhancements have been applied:
- Fixed a compilation problem with GCC 4.7 caused by the wrong order of function declarations. Also removed unnecessary indirections and unused variables.
- Improved out-of-source build in the src-version (for packagers).
- Added virtual destructor in the runtime_wrapper-class in the kernel generator.
- Extended flexibility of submatrix and subvector proxies (ranges, slices).
- Block-ILU for compressed_matrix is now applied on the GPU during the solver cycle phase. However, for the moment the implementation file in viennacl/linalg/detail/ilu/opencl block ilu.hpp needs to be included separately in order to avoid an OpenCL dependency for all ILU implementations.
- SVD now supports double precision.
- Slighly adjusted the interface for NMF. The approximation rank is now specified by the supplied matrices W and H.
- Fixed a problem with matrix-matrix products if the result matrix is not initialized properly (thanks to Laszlo Marak for finding the issue and a fix).
- The operations C += prod(A, B) and C −= prod(A, B) for matrices A, B, and C no longer introduce temporaries if the three matrices are distinct.