Include hardware backend as a link to the actual backend

I propose this clean implementation of the hardware backend in PyNN.
As we think that the hardware backend development should not be part of the PyNN main track, I have implemented a correct link to the directory inside symap2ic where the backend lies.

As such, the code will not be modified even if other submodules are added inside symap2ic, which is a huge advantage, and it makes the two developmenets (PyNN and symap2ic) fairly well separated