Added USGS to the data collection.
Added shell script to setup.py
Added usgs instant and daily values.
Abstracted the USGS code
Modified setup.py to install text scripts.
Made a few tweaks to the IO.
Added filters to site meta IO for USGS.
pep8
merge from master
generalize default_params option
leverage new climata base classes & owslib.waterml

 - load parameter codes directly from USGS
 - clean up unused constants
enable state & county filters
update readme with usgs info
include units in output