fixes issue #236
provide more context information in exception messages
bootstrapping an XML exporter
export video blocks
export section icons
export discussion blocks in XML
add helper methods to ease the XML handling
properly encode data from ISO-8859-15 to UTF-8
use a generic block visitor instead of specialised ones
XML schema defintions

A global XSD file is provided by the infrastructure. Blocks have to
ship their own XSD files for all attributes that are exposed through
the export() method. Thus, the schema can be validated and is
extensible.
move global schema definition file
import XML validation

The XML validator takes an XML input. It then collects all namespaces
that are registered in the document being validated. For each namespace,
the corresponding XML schema definition file is loaded from the local
file system. Then, an additional temporarily existing XSD file is
created that imports all locally found definition files, therefore
validating the input document using all relevant XML schema definitions.
export the HTML block

The export() method in blocks has been renamed to exportProperties()
This makes it clear that it only exposes the metadata of a block. A
new exportContents() method does now exist that exposes the contents
of a block. If the so exposed data is not null, it will be added as
a CDATA section to the block node's content.
escape special characters in XML attributes
export test blocks
added export controller