Add explicit struct @type

Elixir 0.14.3 removed the automatic type defintion for `defstruct`, so
add in the explicit type definition and remove the type annotations to
remove the deprecation warnings.
Update ex_doc
Update elixir compatibility
Add ExVCR for integration testing with weather.gov
Weather.NOAA.Observation.current_raw/1 and .current_xml/1
Weather.NOAA.Observation.current

Get the current temperature and station information.