Whoosh 2.5 performance quickfix

In Whoosh 2.5, fields that can be sorted on need to specify
sortable=True or else take a large performance hit because whoosh
completely dumped its old method of speeding up sorting (automatic
caching) in favor of this brand new one.

These probably don't *all* need to be sortable, but hey.

Note that we now require Whoosh>=2.5 because the sortable keyword arg
didn't exist in exist in earlier versions.
Knock Unicode(256) down to Unicode(79)

MySQL again. Indexed keys have a maximum length of 767 bytes and unicode
is stored as 3 bytes per character. Since 256*3 = 768, that pushes us
just past the limit. Could lower the length to 255 characters, but let's
go further - currently only names and identifers have a length limit,
and if a name or identifier doesn't fit in a terminal then i don't want
to see it.

Fixes issue #123.