parse dst values if they exist, regardless of siteUsesDaylightSavingsTime attribute in waterml v1.1; closes #44
minor waterml docstring fixes
bugfix: error in parsing waterml site properties; closes #45
rename usgs site properties so they are at the top of the site dict itself
minor formatting
add input_file kwarg to nwis.core.get_sites
implement nwis.hdf5.update_site_list and get_sites
implement nwis.hdf5.get_site
cast to object type when replacing nans with None's
df.where() doesn't seem to be doing this implicitly in some cases
misc code organization
use df.combine_first() to merge in updates to sites list
add a test to ensure that the sites table remains unique (doesn't contain duplicate sites)
test that site list updates contain sites that are exclusive to previous and most-recent files
test that updated site attributes (e.g. site['name']) propagate to stored site list