add last_modified attribute to documents

We were testing for it, but we never actually added it...
add crosspost links to documents

This is just the bare minimum of what is desired here. We will
eventually need to be able to link documents by relative path and not by
page URL (page url may be changed based on unrelated things, like a
simple copy/paste of a document).

Fixes #56
remove unused dump method

We just use the object as the underlying hashref it is
reorganize template vars to prefer objects

This does increase some confusion, which I'm loathe to do, but it allows
any future Page/Document object to Just Work with the default templates,
and it solves needing to explicitly add new attributes in various places
in the code before being able to reach them in the template.

Fixes #58
everything used in template should have default

This makes it a lot simpler to check for membership in a hash and/or
loop over an array. Instead of first checking for existence, we can go
right to dereferencing. There is probably a better way, maybe via
helpers...
add crosspost links to the default blog templates

Fixes #56