Snapshot model weights/solver state to HDF5 files.

Snapshot model weights/solver state to HDF5 files.

Summary of changes:
- HDF5 helper functions were moved into a separate file util/hdf5.cpp
- hdf5_save_nd_dataset now saves n-d blobs, can save diffs instead of
  data
- Minor fix for memory leak in HDF5 functions (delete instead of
  delete[])
- Extra methods have been added to both Net/Solver enabling
  snapshotting and restoring from HDF5 files
- snapshot_format was added to SolverParameters, with possible values
  HDF5 or BINARYPROTO (default HDF5)
- kMaxBlobAxes was reduced to 32 to match the limitations of HDF5