Add subprocess32
Use subprocess32 in Python versions < 3.3
Add AstrometryNetTimeoutExpired exception
Add 'timeout' argument to astrometry_net()
Update docstring of astrometry_net()
Add --timeout option
Add Bash completion for --timeout
Update call to astrometry_net()
Use a different message when AstrometryNetTimeoutExpired is emitted
Add --timeout option

This option sets the time limit before Astrometry.net gives up trying
to find the astrometric solution for each FITS image. This is exactly
what the --cpulimit option of solve-field does, but there is a (known)
bug that causes it to be ignored unless the 'inparallel' option is set
in the configuration file (astrometry.cfg). Until this is fixed, use
the 'timeout' argument of subprocess.call() — it was added in Python
3.3, so we need the module 'subprocess32', a backport for use on 2.x.

https://github.com/astrometry/usermanual/issues/9
https://groups.google.com/forum/#!topic/astrometry/gyWYg3bQmZ0

This merges branch 'add-timeout-option'