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. 
I'm not sure this should be hard-coded here.  How is version bumping typically done?  I think the best way to do this would be to
```python
from biom import __version__ as biom_version_string
```
And then set the default in the constructor to ```biom_version_string```.  Is there a reason it needs to be a tuple? If so, then you'll have to parse the string into a tuple, and also allow that the version string might be suffixed with "-dev", since when I do this import I get ```2.0.1-dev```