Applied all suggested changes from sf.net:

- Kernels are generated in build/ and no longer penetrate the source tree. Old kernel files removed. Users interested in ViennaCL-Nightly should consider creating a symlink viennacl/linalg/kernels pointing to build/viennacl/linalg/kernels and run 'make' in build/ after each pull.
- Issue with trans_prod_impl() in GCC 4.7 solved.
- Virtual destructor added in runtime_wrapper class within kernel generator
- Updated documentation for 1.3.1 release