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. 
Agree, this should take as default the current version instead of hard coding a version as default. The benefit of using a centralized version is that it makes bumping version at release much easier. 

...wonder if we should push further forward on (verman](https://github.com/wasade/verman)