Issue #298, Add offset as a configuration option to the collection tile.

This adds a configuration option "Start at item".  With this option two or more collection tiles
can be used to show different slices of the same collection.
Update CHANGES.rst with log from previous commit.
Merge remote-tracking branch 'upstream/master' into issue_298

* upstream/master:
  changelog entry for recent change, pull request 328.

Conflicts:
	CHANGES.rst
Add tests for the "Number of items" and "Start at item" configuration options.
Merge pull request #334 from warpr/issue_298

 Issue #298, Add offset as a configuration option to the collection tile.