Correctly parse tag queries with more spaces or without tags
Revert the tag query change that was introduced in version 2012-08-24

The semantic of a tag query is relatively obscure but also clearly
documented. Each part of the query relates to the result of the previous
parts, tags without prefix add pages with that tag to the result, a tag
prefixed with "+" removes pages without that tag, a tag prefixed with
"-" removes pages with that tag.

After the 2012-08-24 version this behavior was a bit different, here the
query was executed as if all prefixed tags were at the end.

With this change the original behavior is restored.
Assume an implicit query for all pages if no tag query is given, fixes #35

This lets the tag plugin assume an implicit query for all pages if the
query doesn't start with an unprefixed tag. This means that now "+tag1
+tag2" returns the same results as "tag1 +tag2" and "-tag1" returns all
pages that are not tagged with "tag1". This also adds tests for this
behavior. The tagRefine-function now uses the same code as the
getTopic-function except that the result is limited to the specified
pages and that no filters apart from the tags are applied.
Add new sorting key "namespace", closes #69
Add page name fallback for sorting by title

This uses the page name as fallback when sorting by title is selected,
the fallback is generated in the same way as the pagelist plugin
generates the title that will be displayed.
New version 2013-04-14

Changes:
  * New sorting key "ns" which lists for each namespace first all pages
    in the namespace sorted by name and then all subnamespaces sorted by
    namespace name
  * Sort by page name (as displayed by the pagelist plugin) when no
    title exists and the sort key is title
  * In the topic syntax an implicit query for all pages is assumed if
    the query starts with a tag with "+" or "-", this means that "+tag
    +tag2" is now the same as "tag +tag2" and that "-tag1" will return
    all pages without tag1.
  * The tagRefine-function (used by the blog plugin) now also supports
    tags without "+" or "-"
  * Revert the tag query change that was introduced in version
    2012-08-24, before this change the query was executed as if all tags
    with a "+" or "-" were at the end while the documentation actually
    describes that the order matters. Each part of the query modifies
    the existing result set and either adds new pages (tag without "+"
    or "-") or removes all pages with ("-") or without a tag ("+").