Memo artwork and other misc updates

I have a few miscellaneous updates / bug fixes to make which are 
combined in the attached zio (zip) file.

1. calendarcontrol.py - Added a "Today" button which will select 
today's date in the calendar. There was no easy way to get back to 
"today's date" without hunting for it. I wasn't sure about the 
positioning of the button, but I think it is reasonable.

2. calendarentryeditor.py - Changed the previous and next buttons to 
images to match the phonebook entry editor arrows. On the Mac, these 
only show up as 256 color images (Windows is fine). I also noticed this 
difference in the wxPython demo. It seems to be isolated to 
wx.BitmapButton.

3. gui.py / guihelper.py - I made some add/delete images for the new 
Memo tab.

4. phonebook.py - If you press delete when there are no entries in the 
phonebook, an exception comes up. GetSelectedRows() returns [0] if 
there are no entries, and this is not checked for.

5. phonebookentryeditor.py - In the category manager, if you press 
delete when no category is selected, an exception comes up. The 
variable v is returned as '' and this is not caught by the try...except 
block.

Thanks,

Adit