Redirect stdout and stderr to null device unless verbosity != 0
Send 'verbosity' minus one -v flags to Astrometry.net
Update docstring of astrometry_net()
Update help text of --verbose
Ignore by default the standard output and error of Astrometry.net

Redirect the standard output and standard error of Astrometry.net to
the null device unless a value of 'verbosity' other than zero is used.
The number of -v flags sent by astrometry_net() to Astrometry.net is
equal to 'verbosity' minus one: it may be thought of as if the first
-v enabled both stdout and stderr, while the rest of them were passed
to Astrometry.net.

Merges branch 'astrometry_net-ignore-output' into multicore-astrometry
Add support for keyword arguments (**kwargs) to memoize()
Add process shared queue
Move code into function parallel_astrometry()
Add --cores option
Add Bash completion for --cores
Use multiprocessing.Pool.map_async() to do astrometry in parallel
Update docstring of astrometry_net()
Add Queue.clear()
Clear the process shared queue before exiting
Move CONFIG_FILENAME to setup.py
Move CONFIG_PATH to setup.py
Clear the urlretrieve() cache in get_dss_image()

urllib.urlretrieve() does not copy the object if a valid cached copy
of the object exists. This causes get_dss_image() not to download an
image from the STScI Digitized Sky Survey if it has been previously
downloaded — for example, in two consecutive calls to the function.
Fix it by clearing with urllib.urlcleanup() the cache that may have
been built up by previous calls to urlretrieve(), so that we can
download an image for the same coordinates as many times as needed.
Add multicore support for 'astrometry'

Use the multiprocessing module to spawn several Astrometry.net
processes simultaneously and solve the input FITS images in parallel.
By default, the number of worker processes equals the number of CPUs
in the system.

The parallel implementation of 'astrometry' is heavily IO-bound and
clearly follows the law of diminishing returns, as each of the spawned
processes needs access to (potentially the same) Astrometry.net index
files. Nevertheless, tests run on systems with 4, 8 and 16 cores seem
to indicate that, in general, in order to achieve optimal performance
we must use as many CPUs as are available.

This merges branch 'multicore-astrometry'