Dynamically loadable HDF5 filter plugin

HDF5 1.8.11+ supports "dynamically loadable filter plugins" (http://www.hdfgroup.org/HDF5/doc/Advanced/DynamicallyLoadedFilters/HDF5DynamicallyLoadedFilters.pdf). Following that recipe, I've made a plugin for the blosc filter.

I needed to do this anyway for my own work and thought I would put in a pull request upstream to see if there is interest in having this in blosc.  I'm sure this isn't up to standard for merging yet: you will probably want to change the format and content of the headers and I put in the bare minimum for the README.