Add text property to hippo:resource to store extracted text version

Add a text property (of type binary) to hippo:resource. This property will store the extracted version of a resource.

Rationale: extracting text from pdf during the save of a document blocks other searches (JackRabbit behavior). To avoid this extract the text during (or more precisely right after) the upload. (As opposed during the save of the document) Later on we will change the hippo:resource plugin to use this property.