Show the completed percentage as each finding chart is downloaded

The download of the finding charts, especially the establishment of the network connection, takes sometimes longer than one would expect. Using [urllib.urlretrieve()](http://docs.python.org/2/library/urllib.html#urllib.urlretrieve) we can take advantage of its hook function to display and keep updated the percentage of the download that has been completed so far. A message such as `Downloading M31.jpg: 0 %` is shown when the download starts, and the percentage is updated every time a new block of data is read.