Added USGS to pyoos collectors/parsers

Pretty straight forward - all additions, no modifying of existing files (except .gitignore)

```
from pyoos.collectors.usgs.usgs import USGSCollector

usgs = USGSCollector()

# get a station collection by specifying bbox
station_collection = usgs.get_stations_by_bbox(north,south,east,west,[period_days,[period_hours,[startDT,[endDT]]]])

# get station collection by specifying state code (USPS)
station_collection = usgs.get_stations_by_state(state_code,[period_days,[period_hours,[startDT,[endDT]]]])

# get station by specifying the site id
station = usgs.get_station(id,[period_days,[period_hours,[startDT,[endDT]]]])
```