Use multiprocessing module to download the finding charts in parallel

As suggested by Juan de Dios Santander, use a multiprocessing pool of
workers to download multiple finding charts simultaneously. The number
of workers in the pool is equal to that of CPUs in the system (e.g.,
if there are four cores on the machine, charts are downloaded from the
SDSS server in batches of eight).
Do not show completed percentages as finding charts are downloaded

Now that the finding charts are downloaded in parallel we cannot
display the percentage of each download that has been completed so
far, as multiple downloads would update the percentage at the same
time. We need to revert to the original output, showing a slightly
more detailed information so that the user always knows to which
finding chart each message refers.
Update expected output — charts downloaded in parallel
Merge pull request #7 from vterron/master

Add parallel downloading for increased speed. Closes #4