Ulmo

- core API functions can be called from main dataset modules
- support python 2.6

- add ghcn daily dataset
- add preliminary support for CUAHSI WOF
- add sphinx documentation
- clean up tests
- make pytables an optional dependency; pytables-related functionality is just
disabled if pytables can't be found

- bugfix: broken usgs.pytables last_refresh handling

- turn on compression for pytables cache files
- minor bugfix and test cleanup

- initial public release as ulmo (formerly pyhis)