Provide adapter to manipulate DocumentModel as a blob

There are several place in the code where we need to manipulate the DocumentModel as a blob :

• in all download links
• in the preview addon
• in all file oriented interfaces (WebDav, Flex filemanager ...)
• ...

Each time, there is ugly code that looks for file:content.
=> This needs to be encapsulated

Furthermore some services (like tranformers or Tiling) take as input something that is homogene to a blob.
In these case, using a blob instread of a Adapter DocumentModel makes us loose information : there is not way to compute a cache key based on the blob.

We need to define :

• a BlobHolder interface
  (that can be implemented on top of DocumentModel or on top of other resources)
• a pluggable adapter factory to manage how BlobHolder is defined for each DocumentModel Type