Create new builder for building sitemaps 1 at a time

[jwoertink]

Right now when you're building the sitemaps, they're all just shoved into memory and then you call store to build and send off the whole thing.

If your site has a lot of sitemaps, this could be a huge memory bloat. It might be better to just build them one at a time, and ship each one off as it's built.

It appears a previous PR did this #38 and then #39 seems to fix a possible memory leak