resolves #411 add DocBook 5 backend
add xmlns to root element in docbook45 backend
noxmlns attribute disables xmlns in DocBook 4.5
Merge pull request #544 from mojavelinux/docbook5

resolves #411 add DocBook 5 backend
resolves #556 print warning/error messages using warn
Merge pull request #558 from mojavelinux/print-messages-with-warn

resolves #556 print warning/error messages using warn
rework Block constructor, optimizations

- use options array to pass Block properties to constructor
- use left shift operator to add blocks to parent
- allow paragraph text to be passed to Block as string
- pull content_model up to AbstractBlock
- add :empty content_model
- pull up some regexps to constants
- update rdoc
cleanups and optimizations

- only normalize option attributes for top-level document
- only set dynamic attributes on top-level document
- set dynamic attributes before loading reader
- optimize several non-nil checks