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.