find HDF5 libraries in CMake
get hdf5 file writing working
write label data to hdf5
io: now log text file to specified filename
CMakeLists: speculative fix for linker error

Use generic HDF5_LIBRARIES variable rather than binding-specific one.
Merge pull request #8 from rjw57/depth-dump

log to a (hardcoded) text path and (hardcoded) h5 path