Dear all,

here is a patch implementing category filtering in the agenda.

The patch is not 100% clean wrt documentation, but I throw it
now to get some feedback and some testing done. 

Press "<" in the agenda to filter by category.

Press "< <" to filter by the category of the entry point.  
Another "/ /" removes the filter.  "/" + " " filters by the 
default category ("General") since everything is in a category.

I find this last function quick and useful, so I also implemented 
it for tag filtering: pressing "/ /" filters by tags from the entry
on the current line.  A second "/ /" removes the filter.

Let me know if the category filtering works ok with you, and 
if you find the new "/ /" behavior useful.  

There is one limitation for now: it does not combine with Effort
filtering.

Thanks,