Multiple data entries per page

Adds the ability to have multiple data entries per page (addresses issues #28 and #47).

This is done by extending the database with a entry field. The entry field is handled in mostly the same way as the page id (i.e. %entryid% behaves the same as %pageid%).

The entry identifier is entered as follows:

```---- dataentry [Identifier, like a section title] entryclass ----```

The [ and ] can be left out if the entry should have no identifier (i.e. it is the page entry itself). This makes it backward compatible, and those that do not use this feature will not be forced to change anything.


I am, of course, open to suggestions, comments, criticism and improvements.