INT-2882 Upgrade DocBook Reference Plugin to 0.2.6

For reference see: https://jira.springsource.org/browse/INT-2882

* Verify spacing
* Ensure all source code samples are typed: e.g. <programlisting language="xml">
* Ensure source code fits space in PDF format
Merge pull request #707 from ghillert/INT-2882
INT-2958: XSD: move 'charset' from common attrs

`charset` for `<object-to-string-transformer>` was placed in the common `inputOutputChannelGroup` attributeGroup.
`charset` wasn't available  within `<chain>`.

* Move 'charset' from `inputOutputChannelGroup` to the new `specialized-transformer-charset-aware-type` complexType
* Add test for 'charset' within `<chain>`

JIRA: https://jira.springsource.org/browse/INT-2958
Merge pull request #764 from artembilan/INT-2958