Added support for shared lzf library to be used with HDF5's dynamic filter loading

HDF5 1.8.11 support dynamic loading of filters by putting a shared library into /usr/local/hdf5/lib/plugin or by HDF5_PLUGIN_PATH environment variable. 

I copied basically the `lzf_filter.c` and added  two functions which are required to by the dynamic filter floading: 

`H5PL_type_t   H5PLget_plugin_type(void) {return H5PL_TYPE_FILTER;}`
`const void    *H5PLget_plugin_info(void) {return H5Z_LZF;}`

to compile it you just need to run: 

`h5cc -O2 -fPIC -shlib -shared  lzf/*.c lzf_plugin.c -o libhdf5lzf.so` 

I tested it with `h5dump` and `h5ls` and it worked.