Incorporated validation-status port

New output port for dtbook-validator and nimas-fileset-validator called
validation-status. It outputs validation status XML
(http://code.google.com/p/daisy-pipeline/wiki/ValidationStatusXML).
NIMAS and DTBook Validators check for well-formedness of XML

Validators check that input documents are well-formed.

Split validator structure into many parts to facilitate this.
Bugfix

NIMAS/OPF validation now correctly detects the presence of MathML in
the associated DTBook documents.
Merge pull request #15 from daisy-consortium/check-well-formed

Check well formed