New way to deal with incomplete dt columns. Leaving the NA's as they come, and adding a date column.

Fixes #140. So now, if a column that is modernly displaying date and time, it still gets returned as a POSIXct dateTime. If some of those columns are only dates, a time of noon...with the median time zone offset...is added, and a new column is created to indicate a TRUE/FALSE date flag.

Fixes #102.

Fixes #106

Fixes #101

Updated documentation to change NWISweb to USGS (NWIS) to help non-USGS people understand the connectiona a bit better.

Took out qwdata option for readNWISdata function. That would need to be sent to waterdata instead of waterservices.