Add preliminary BoxLib based backend.

This PR adds preliminary support for a state backend based on [BoxLib](https://ccse.lbl.gov/BoxLib/index.html).  To use this backend you need to install PyBoxLib:

    git clone https://ccse.lbl.gov/pub/Downloads/BoxLib.git
    cd BoxLib/Src/Python
    python setup.py install --user

Next, create a symlink in `clawpack/clawpack` to the `boxclaw` module:

    cd clawpack/clawpack
    ln -s ../../pyclaw/src/boxclaw boxclaw

Now you should be good to go!

    cd clawpack/pyclaw/examples/acoustics_1d_homogeneous
    python acoustics_1d.py use_boxlib=True