Speed up startup and introduce progress bars

Ahoi!
Commit cea68db introduces progress bars while scanning for files and a spinner while initially discovering albums. This fixes issue #102 .
Commit d69bc98 shifts the parsing of the exif-data for sorting to later in the process, so that, if you don't want to sort, things are sped up dramatically (from ~3 minutes to some seconds on my 15k image library).  Additionally this way the image count is known and a progress bar can be displayed.
Yes, it is. It's only been  shifted from the constructor (at a time when the total amount of images is unknown) to a time when the total amount is known so we can display a progress bar. However I wanted to keep the interface the same, so I added this optional argument.