ENH: reading NEXRAD (WSR-88D) Archive 2 files.

Functions for reading NEXRAD (WSR-88D) Level II file stored at NCDC:
http://www.ncdc.noaa.gov/
ENH: reading of thredds NEXRAD level II data
MAINT: refactor of cdm.py (thredds.py)

Renamed thredds.py to cdm.py.
Refactored and renamed function in cdm.py.
Added function in cdm.py to documentation.

Conflicts:

	pyart/io/__init__.py
MAINT: renaming NEXRAD io files
MAINT: WIP refactor of nexrad reading
MAINT: WIP refactor of nexrad reading part 2
MAINT: WIP refactor of nexrad reading part 3
MAINT: finished refactor of nexrad reading
TST: Unit tests for NEXRAD modules

* Three small NEXRAD files and associated script in testing/data
  needed for unit tests.
* Three units tests (test_nexrad_archive.py, test_nexrad_cdm.py and
  test_nexrad_level2.py)