docstrings, general cleanup

Also moved the port into config. I'm thinking about a way to make R read
from the same file, but that's a touch I don't have time to think about
yet.
data limiting

Right now you can only limit by dream number; the next step is to make
it general and work for any field.
limits: can limit any int field
start of categorical filtering

You can build a place query, but it doesn't do anything yet.
filter by categorical data supported

We still can't filter properly by theme, but that's an architectural
difficulty that's not related to the filtering system -- to get that
working, we have to split the themes up. (The filter works just fine --
but the options aren't what you would expect, as if there are multiple
themes in some of the data you have to AND all the matching ones in
manually.)

A good next step, I think, is getting a way to create a facet or
graph-on-top-of-graph showing unfiltered vs. filtered data. Then it will
become possible to see the changes that various subsets make, and that
should make things a lot more interesting.

If there could be a way to re-graph the last graph from any menu, that
would make it easier to play with subsets.

Oh, also, we still need to support filtering by date. Shouldn't be *too*
difficult, but will require diving into the datetime docs.