ENH: Add version as attribute of Table

fixes#510 

Version should be an attribute of the table.  On init it currently defaults to (2,1) for future releases it only needs to be changed in one place now. 
Yeah, I added a couple functions to QIIME that were later ported over to
verman:
https://github.com/biocore/verman/blob/master/verman/__init__.py#L98-233

On (Jul-23-14|16:01), adamrp wrote:
> > @@ -83,9 +83,20 @@ def generate_subsamples(table, n, axis='sample', by_id=False):
> >          yield table.subsample(n, axis, by_id)
> >  
> >  
> > -def get_biom_format_version_string():
> > -    """Returns the current Biom file format version."""
> > -    return "Biological Observation Matrix 1.0.0"
> > +def get_biom_format_version_string(version=None):
> 
> @ElDeveloper, is there a way to get the SHA of the commit, in the case that it's dev?  If so, that would be excellent to include here, too!  I think I have seen an example of this when I execute ```print_qiime_config.py```
> 
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> https://github.com/biocore/biom-format/pull/512/files#r15321621