Use less =item in POD in favor of =head*

By using =item for long sections we don't get those things included in
TOC's on e.g. metacpan.org. Just make them =head* instead, mostly
=head3's.

I'm fairly sure I got the intended heading level in sereal_spec.pod
right, but it's possible that I didn't.
Sereal::{Decoder,Encoder}: clarify that no options are on by default

I kept looking through the docs trying to figure out what the default
of these options was, clarify that explicitly in the docs.