Modularized the <head> section

Include the <head> code from external files instead of hardcoding it
inside Page.ss. This makes easier to modify the head in subtemplates or
to extend Page.ss.
Move cache logic into Page.ss

This should make the maintenance easier. The performance gain, if any,
is negligible (tested, not guessed).
Revert "Move cache logic into Page.ss"

This reverts commit 4d7a71077ffa185fea7b56092042be8a1934f03a.

Having <% cached %> blocks in Page.ss prevents the uncaching of data in
the included templates, defeating the whole purpose of "dynamic site".