Disable "Convertible" and make compatible with Octopress

I have had it with these monkey-patching methods in this Monday-to-
Friday language!
Add `slug` getter to Pages

Also made tiny changes only specific to Octopress
Move `slug` function to inside `StaticComments` module

Now we can use it elsewhere.

Also make sure any `id` passed to `self.find_for` is slugged (just to
make sure if the `id` was defined in the YAML, it makes sure to pick up
all related comments.
Merge branch 'dev'