Move non-public RmDiskInfo and RmPartInfo from .h to .c
Remove unused size_table from RmFileTables
Added full support for mtime filtering.

Sample session:

    $ echo 'xxx' > /tmp/one # for later demonstration
    $ rmlint /tmp -O stamp:last-update.stamp -c stamp:iso8601
    <finds dupes in /tmp normally>
    $ cat last-run.stamp
    2014-09-07T18:16:21+0200
    $ rmlint /tmp -N $(cat last-run.stamp)
    <finds no dupes>
    $ echo 'xxx' > /tmp/two
    $ rmlint /tmp -N $(cat last-run.stamp)
    # Duplicate(s):
        ls /tmp/two
        rm /tmp/one

* -N (--newer-than-iso8601) uses ISO8601 Timestamps, -n (--newer-than) uses
  plain UTC-seconds-since-epoch.
* If files with newer mtimes are found, all files with same size are considered.
* The stamp formatter should make scripting tasks very easy (i.e. watch a
* Download directory for new files)