replace util.parse_datestr with a set of more general functions for converted to dates and datetimes
change usgs.nwis date_range parameter to use explicit start/end or period parameters
update docs with a note about the acceptable date/datetime formats
bump to version 0.4.0-dev due to backwards incompatible change and add note to changelog
clean up usgs.nwis tests: move files to a separate directory and rename usgs_test.py -> usgs_nwis_test.py
cleanup ncdc.ghcn_daily tests: move test files into their own directory
move cuahsi.wof test files to their own directory
update usgs.nwis docstrings to note that start/end dates and periods can be None
format `None` consistently in usgs.nwis.pytables docstrings