DTBook to ODT converter

Taken from github.com/sbsdev/sbs-pipeline commit 036e33e, and all the
SBS-specific parts filtered out.
Add an option `page-numbers-float`

which can be disabled to render page numbers exactly where they appear
in print, splitting up paragraphs when necessary.
Give code that handles pagenum and note skipping a higher priority

to reduce code duplication
Add XProcSpec tests for content.xsl
Some pagenum related fixes
Update the README to contain the dtbook-to-odt script
dtbook-to-odt: test with version 1.8 of pipeline

also:
- use pax-exam-helper
- use x:script feature of xprocspec
dtbook-to-odt: put non-script XProc utils in library.xpl
Exclude (temporarily) PAX Exam tests
Add dtbook-to-odt to BoM
Merge pull request #33 from daisy/dtbook-to-odt

Closes #33