blog source -> store

"store" is far more appropriate, since it's the Statocles::Store object
for the application

Fixes #88
fix pre and code wrapping

Fixes #89
add coercions for Theme and Store

Since these objects are mostly just paths, why not coerce the real
object from the simple path string? It makes the initial setup a lot
easier.

The only tradeoff is that it's not as obvious what is happening in the
site config file, so it may discourage others from creating their own
Theme or Store.

Fixes #69
change theme source_dir -> path

Internal consistency is good.

Fixes #90
add sitemap.xml generation

Right now this is absolutely not customizable by applications in any
way. In the future, we may add Page-level metadata so that apps can
decide what the sitemap looks like.

I'm not sure I'm using Mojo::DOM to create these things correctly.

Fixes #61
add a simple robots.txt

The main purpose of this is to point search engines at the sitemap.xml,
but in the future we'll be able to mark documents as noindex or
something.

Fixes #62