Added support for rendering flat numbers in addr:flats

[derickr]

This patch adds support for rendering addr:flats instead of addr:housename - which makes a lot of sense if you see things like:

The nodes all have addr:flats, addr:housename and addr:street - but the housename shows up 20 times or so.

With this patch, it renders like:

I've made the number one point smaller than real house numbers, but I'm happy to have it render differently - just thought it would be nice that it shows up. It makes a lot of sense in this area as there are so many instances. Annoyingly, it requires a new column in the database.

[pnorman]

Closing. Still working out what tags will be in columns vs hstore, but addr:flats is extremely unlikely to be in a column, so this PR would need complete rewriting at that time. Same as #605 (comment)

Some things to consider when it comes time

• what if all the addresses have a flat? Then housenumber/housename wouldn't appear at all. Is this a common situation?
• flat numbers shouldn't be a higher label placement priority than housenumbers. the right approach is probably to filter out flat numbers from the housenumber/housenamelayer, this way if there is a conflict between a nearby housenumber/housename and a flat number, the latter gets displayed.
• Should all three address layers be combined into one?

I suggest adding comments on #264