Dont fail if layered shallow water solver won't compile

I'm trying to fix #63, but this doesn't completely do the job.

I've made it so that if the layered Riemann solver didn't compile -- but all others did -- then you still get all the others on import.  I've also stopped providing default lapack/blas flags, and now we only pass the flags when compiling the layered solver. 

What's lacking -- and this is the most important part -- is how to keep the installer from exiting when the layered solver fails to compile.  Putting the config.add_extension() call in a try block does no good.  Numpy.distutils has defeated me here.  Any ideas?