Fix failing hdf5 tests.

Some of these tests were failing for me for a while--I figured it was a problem with my h5py installation but upon closer inspection it seemed to be a problem more with the tests and Unicode issues.

I don't know why these weren't caught sooner except that maybe most of the test bots don't have h5py installed.

This fixes the tests for now but there are some broader issues that need a look involving Unicode support.  Currently h5py does not support numpy 'U' arrays, though it has no problem taking utf8-encoded bytestrings.  So we might have implement some sort of hack to loop through table columns and convert any unicode arrays to byte arrays before shoving them into the hdf5 dataset.
As pointed out the HDF5_SIGNATURE should be a byte string, so let's make it so; 2.6 and up support bytes literals
Merge pull request #641 from iguananaut/fix-hd5f-test

Fix failing hdf5 tests.