Making keywords optional, case-sensitive tags, viewers now work on CygWin, many fixes

Hi Edward,

here is a batch of changes:
1) I made tags to be case sensitive when added or removed from the document in 'sort' function. If you don't agree let me know and I will make it configurable.
2) I changed vim commands MinionOpen and MinionOpenAll (new command) to behave the same way as minion shell command line. It works very well for me, I hope it will for you too.
3) Commands 'dates' and 'recent' now don't require keywords. If none given, 'all' is assumed.
4) Minion now works on CygWin including viewing non-text files. It launches Windowz viewers where necessary.
5) Added action to delete note to 'sort' command
6) Changed leading characters to add or remove tags in 'sort' command. Tags can now start with '@' character (which makes it work very nicely with vim vim-notes plugin).
7) A lot of bugs found and fixed.

Let me know if any questions or concerns.

Cheers

-Pavel