new app: scanning cabinet

WARNING: this app is still experimental, and even its data schema might
change. Do not use in production.

This change adds a Camlistore-based port of the scanning cabinet app
originally created by Brad Fitzpatrick:

Some of it is inspired from the App Engine Go port of Patrick Borgeest:

The data schema is roughly as follows:

-a scan is a permanode, with the node type: "scanningcabinet:scan".
-a scan's camliContent attribute is set to the actual image file.
-a scan also holds the "dateCreated" attribute, as well as the
"document" attribute, which references the document this scan is a part
of (if any).

-a document is a permanode, with the node type: "scanningcabinet:doc".
-a document page, is modeled by the "camliPath:sha1-xxx" = "pageNumber"
relation, where sha1-xxx is the blobRef of a scan.
-a document can also hold the following attributes: "dateCreated",
"tag", "locationText", "title", "startDate", and "paymentDueDate".

Known caveats, in decreasing order of concern:
-the data schema might still change.
-the scancab tool, to actually create and upload the files from physical
documents, is practically untested (since I do not own a scanner).
-some parts, in particular related to searches, are probably
-the usual unavoidable bugs.

Change-Id: If6afc509e13f7c21164a3abd276fec075a3813bb
17 files changed