Don't include empty query parameter in URLs

The spreadsheets API endpoint returns an HTTP 400 error with a
message of "Invalid query parameter value for sq." when the
``sq`` query parameter is empty.  The default case for Tabletop
is an empty query.  Only add the ``sq`` parameter to the generated
URL if ``this.query`` is not empty.

Addresses https://github.com/jsoma/tabletop/issues/45
Detect worksheet ID in both old and new sheets.

Tabletop.loadSheets looks at the ``data.feed.entry[i].link``
elements to determine the worksheet ID for each worksheet. With
old sheets, the worksheet ID can be parsed from the fourth link
element.  However, with new sheets, there are five entries in
``data.feed.entry[i].link``.

In either case, we can parse the worksheet ID from the last
element in ``data.feed.entry[i].link``.  So, use this last element
instead of hard-coding access to the fourth (index 3) element.
Merge pull request #48 from ghing/support-new-sheets

Fix determination of worksheet ID for new sheets