added extract text from presentations

bigbluebutton-web/grails-app/conf/spring/doc-conversion.xml

...igbluebutton/presentation/ConversionMessageConstants.java

...bluebutton/presentation/DocumentConversionServiceImp.java

.../java/org/bigbluebutton/presentation/TextFileCreator.java

...ton/presentation/imp/PdfToSwfSlidesGenerationService.java

...presentation/imp/SwfSlidesGenerationProgressNotifier.java

...rg/bigbluebutton/presentation/imp/TextFileCreatorImp.java

...org/bigbluebutton/api/messaging/NullMessagingService.java