rename documents from '.yml' to '.markdown'

The file is Markdown. The front matter is YAML, but the front matter
could potentially go on any kind of file, including templates (.ep),
HTML (.html), plain text (.txt), and etc...

This breaks every Statocles site currently out there. You can use:

    find site -name '*.yml' | rename -s '.yml' '.markdown'

to rename all your files correctly.

Fixes #73
rename statocles site documents to .markdown
require frontmatter to begin with '---'

In order to have frontmatter, the first line of the file must start with
'---'. This will make it easier for us to support frontmatter in other
files in the future, and maybe on a per-section basis...

This is another breaking change, but all the documents produced by
Statocles have always had that first '---'. Only files written from
scratch would not have them.
cache file store's realpath to fix race condition

If our store's directory disappears before we can clean ourselves up (if
it's a temporary directory and the tempdir gets deleted before the
store), trying to find the realpath will throw an exception.

Additionally, when we're in global destruction, we don't care about the
store locations anymore. This prevents spurious warnings about
uninitialized values in hashes if our path object got cleaned up before
we did.