configure: Add --enable-content-hashing arg

Issue #24.
build: Add support for optional content hashing of CSS

Issue #24.
build: Add support for optional content hashing of JS

Issue #24.
www: Center d1-picture in divs
build: Use HAVE_WWW_ROOT to determine local

Don't use ENABLE_HTTPD, as it's possible that httpd could be enabled but
no www root was found.
build: Remove redundant test

We don't need to check for the presence of the www_root at install time
anymore since we do this at configure time.