Added capability to retrieve all documents in a database/view

A new XML configuration option <si-couchdb:inbound-channel-adapter all-documents='true'
controls whether the adapter polls for changes or retrieves all documents.
With the integer attribute all-documents-limit (default 10), one can control,
how many IDs are retrieved from the DB with one GET request.

This functionality is useful for feeding all documents in a view into
a Spring Integration workflow, and the database-url should either be a
view URL e.g.

http://host:5984/dbname/_design/mydesigndoc/_view/myviewname

or an 'all_docs' URL:

http://host:5984/dbname/_all_docs

(cf. http://wiki.apache.org/couchdb/HTTP_Document_API#all_docs)

Note that behinde the scenes, a CouchDbAllDocumentsMessageSource is
instantiated instead of the CouchDbChangesPollingMessageSource when
all-documents is true (default value is false for backward-compatibility).