exif: Make small performance improvements.

However this is temporary. The real version will do batch
processing.
exif stuff: Use the -php option and eval to retrieve file data.

This not only gives us a cool nice option that doesn't require
string processing, but also prepares us for the batch processing
in future.
exif batch processing: Implemented.

We go over the file list and prune it (excludes). Then we create
a string of all files we want to process in a batch, separated by
double quotes and spaces. This is fed to exif which returns a nice
array of arrays in php format. For each file, we get the description
and the date (and some other data) and print our table.