BUG: MdvFile looks at first field header
TST: small example mdv and netcdf files

These example files will be used to test the Py-ART io modules.
The scripts used to create and visualize these example files are
included in the testing/data directory.
TST: moved current tests to system_tests folder
TST: unit tests for io/mdv.py module
TST: unit tests for io/netcdf.py
TST: unit tests for graph modules
TST: unit tests for map modules
ENH: reading of Sigmet/IRIS files
MAINT: sigmet refactor and docs
Merge pull request #68 from jjhelmus/master

ENH: Reading of Sigmet/IRIS files
TST: small sigmet ppi and rhi files
TST: io/rsl.py test
TST: graph/plot_rsl.py tests
TST: unit tests for io/grid.py module
BUG: off by one error in phase_proc.py
TST: example single ray ppi radar
TST: unit tests for correct/phase_proc.py module
BUG: off by one error in attenuation.py
TST: unit tests for correct/attenuation.py module
TST: sample interp sonde and aliased radar