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.
This will break if the exif property is used in the theme and if the date property is not used (no sort by date).
`self.exif` and `self.raw_exif` also need to be properties that can be loaded lazily.