switching to __doc__ in another tab

Hi,

Sbuffer doesn't really work with tabs, so I added support for switching using "drop". Unfortunately that requires vim compiled with gui support, works great even in terminal though, because of that I didn't change the default behaviour but introduced a new variable g:pydoc_use_drop

I think it's nice to have this options for users who prefer tabs (such as myself, indeed)

(this description is way longer than the patch ;-) )

Best of luck :-)
Lukas