#14 handling of copying files

[hlipka]

A file should not be copied if
- source and target have the same size
and
- the source is older than the target
Currently, copying files takes much longer than
processing all transformations (if have many archive
files for software releases). This feature would allow
for only updating changed files.

Also, I would like not to be required to specify the
target name for a file to be copied - it is the same as
the source name in all cases (I think this would be
nearly always the case).