ENH: ARM time vars can be written with write_cfradial

* added `arm_time_variables` flag to write_cfradial function which
  when True will create netCDF files with base_time and time_offset
  variables
Merge pull request #122 from jjhelmus/master

ENH: ARM time vars can be written with write_cfradial
ENH: added _debug_info function.

* The _debug_info function can be used to check on dependencies, versions
  and if RSL is installed and linked to Py-ART.
Merge pull request #123 from jjhelmus/master

ENH: added _debug_info function.