2nd commit
Add functions for downloading GFS/NAM/RAP data

New python modules:
- get_nwp_data.py
  New functions:
  * get_latest_init()
  * get_gfs()
  * get_gfs_mix()
  * get_nam_rap()
  * get_nam()
  * get_rap()
- utility.py
  New functions:
  * remove_old_dirs()
  * get_hostname()

External perl scripts from NCEP:
- get_gfs.pl
- get_grib.pl
- get_inv.pl