add easyconfig for foss/2015a + HPL for testing

Proposal for the `foss/2015a` "common toolchain", consisting of:

* GCC version 4.9.2 (latest to date, Oct 23rd 2014)
* OpenMPI version 1.8.3 (latest to date, Sept 26th 2014)
* OpenBLAS 0.2.13 (latest to date, Dec 3rd 2014)
* FFTW 3.3.4 (latest to date, March 16 2014)
* ScaLAPACK 2.0.2 (latest to date, May 1st 2012)

Latest available versions of all components were selected keeping in mind that they are required for several important features, including (full) C++11 support, OpenMP v4.
This tips the scale in favour of most recent releases rather than being conservative and going for the last update of the previous major release.

Results from testing this toolchain by rebuilding existing `goolf`/`foss` easyconfigs with `foss/2015a` will be posted soon.