Test the error message instead of a predicate

This test was broken prior to commit 370c4bd but
didn't fail because the test harness didn't test for
the right error.
fix rendering of column by `srcloc->string`
at-exp reader: fix source locations in `read` error message

A change in `raise-reader-error` to use `srcloc->string` caused
tests to fail because `srcloc->string` doesn't show location
information if the source is #f.
Scribble->Markdown titles are now rendered correctly. Sections have been shifted by one level (which also looks better on Github).

Markdown: fix defmodule to avoid printing the package in the example

fix markdown example file to make test pass
enable scribble tests for DrDr
history for 2htdp teachpacks
apply signature in productbuild, not pkgbuild

Merge to release branch.
remove extraneous CRLF after message-body

RFC2616 section 4.1 (http://www.ietf.org/rfc/rfc2616.txt):
"... an HTTP/1.1 client MUST NOT preface or follow a request
with an extra CRLF."
scribble: fix a test to make it independent of the enclosing package