added weather data overlays

* only one overlay at a time
* currently only for Americas via http://mesonet.agron.iastate.edu/ogc/
renamed overlays to make them shorter
small tweaks

* a few commends explaining the code
* removed timestamp being added to tile files, their servers provide
  decent expiration headers, should work fine to get the latest
added METOSAT MPE data
Merge branch 'feature-weather'