Added Functionality for NOAA GHCND

Hi Scott,

Here are the R functions I wrote for working with the NOAA GHCND data. They are intended to be performed iteratively, in this order: ghcndGetInfo, ghcndFormatInfo, ghcndFilterInfo, ghcndMapInfo, ghcndGetData, ghcndFormatData, and ghcndRunStats. Let me know if you have any questions.

Cheers,

Adam