Adding tests for pure vexcl/compute vector addition

@kylelutz: Boost.Compute compiles its kernels every time an algorithm is
called. This leads to a considerable overhead, which could in principle be
avoided by caching the compiled kernels in static objects.

Here are timings of the benchmarks:
```
$ time ./vecsum_vexcl
1. Tesla K20c

real    0m0.555s
user    0m0.051s
sys 0m0.432s

$ time ./vecsum_compute
Tesla K20c

real    0m1.395s
user    0m0.659s
sys 0m0.632s
```