#51 Validator doesn't report empty dtb:uid in dtbook

[Romain Deltour]

The spec mentions that the dtbook must have a dtb:uid and its content must match the content of the dc:Identifier of the OPF, which can't be empty.

* The validator should report empty dtb:uid values.
* The Narrator should include a dtbookFix for this
* ZedVal fails with "bad character content for element", the message could be more explicit.

[Romain Deltour]

sample dtbook with empty dtb:uid

[Linus Ericson]

The validator now reports empty dtb:uid values, and a dtbookFix has been added to the repair category that creates an auto generated value.