Added file distinction based on metadata (mtime, mode, owner)

[nbeisert]

fixes #123 fixes #131

description of changes: added functionality to distinguish files based on modification time, file mode, ownership (uid/gid).

considerations:

• no command line switch has been added yet (functionality time/mode/owner always activated for the time being); will add command line options depending on desired implementation.
• internal comparison will always distinguish additional attributes (next to file size and buffers); if no distinction of files due to time/mode/owner is desired, the corresponding attributes are set to 0 upon initialisation such that all files appear identical with regard to these data (reasoning: it would take a lot of administration to carry around the values of command line options throughout the processing unless they are saved in global variables; overhead should be minimal because comparison is invoked on files of identical size only).
• some comparison methods were moved from Rdutil to Fileinfo; this simplifies direct access to (additional) attributes; it also appears natural to have these methods there.
• will collaborate well with rsync when mtime distinction is activated.