Set description in Picasa export only if it is defined

Otherwise Picasa displays filenames as photo descriptions, which is not nice in exported albums
Use Google's terminology in variable names - title/summary, instead of caption/description

Title is the filename and summary is something that is displayed along with the photo on the web (xmp title).
Merge pull request #315 from angryziber/master

Improve Picasa export: do not set irrelevant summary for photos consisting of a filename, keep it empty if xmp title is not defined instead