ENH: Reading in gzip and bzip2 files.

The contents of gzip and bzip2 files can be detected and read using
the pyart.io.read function.
Merge pull request #199 from jjhelmus/read_compressed

ENH: Reading in gzip and bzip2 files.
BUG: Correct scan_type for NOAA HH Sigmet files.

The scan_type for radar files from the radars on the NOAA Hurrican hunter
P-3 aircraft are correctly set using the antenna_scan_mode parameter in the
Sigmet file header.
Merge pull request #200 from jjhelmus/bug_p3_scan_type

BUG: Correct scan_type for NOAA HH Sigmet files.
BUG: scan_type properly determined when reading file with RSL.

* The scan_type is detemined by examining the RSL Radar header scan_mode
  element.  This resolves some issue where UF files are read as the incorrect
  type as the sweep elev and azimuth are no set correctly.
* The fixed angles are set from the fix_angle element of the first RSL Ray
  in each sweep using the new get_sweep_fix_angles method of the Volume class.
Merge pull request #202 from jjhelmus/bug_rsl_scan_mode

BUG: scan_type properly determined when reading file with RSL.
BUG: Support for writing unicode in cfradial.py

* The time_reference variable defaults a string type.
ENH: Support for writing moving platform Cf/Radial files.

This does not add support for writing moving platform data which contain
moving platform geo-reference variables (heading, roll, pitch, etc.)
BUG: RadarDisplay can plot moving platforms.

Plotting of moving platform radars is supported using the RadarDisplay
object but the visualization does not take into account the movement
of the platform when generating plots.
ENH: write_cfradial now includes georef variables.

Any geo-refernce variables (heading, roll, pitch, drift, rotation, and tilt)
will now be written out to a Cf/Radial file when using the `write_cfradial`
function.
Merge pull request #203 from jjhelmus/cfradial_issues

Support for reading, writing and visualization of moving platform Cf/Radial files.