store the omx version as bytes

When storing attributes, PyTables stores str as pickled objects, not the raw bytes.  To store a simple string that non-python HDF5 implementations can recover, it needs to be written as bytes instead.

Author: jpn--

api/openmatrix/__init__.py

@@ -8,7 +8,7 @@
 
 # GLOBAL VARIABLES -----------
 __version__ = '0.3.3'
-__omx_version__ = '0.2'
+__omx_version__ = b'0.2'
 
 # GLOBAL FUNCTIONS -----------
 def open_file(filename, mode='r', title='', root_uep='/',