fix (webapi): Update a resource's lastModificationDate when one of its values is changed

[benjamingeer]

Fixes issue #9.

[benjamingeer]

While doing this, I realised that if a resource has more than one lastModificationDate (which should never happen), it can cause SPARQL updates to insert duplicate data. I opened another issue for this, #32.

[tobiasschweizer]

In the SPARQL templates that handle adding values or new versions of values to a resource, you delete (Delete clause) and recreate (Insert clause) the resource's lastModificationDate so it reflects the time when the last modification has been made (as the name suggests) :-)

Why does this have to be included in the Where-clause of the templates? It is optional, so it won't affect the matching to a specific resource.

 @* Get the resource's last modification date, if it has one, so we can update it. *@

    OPTIONAL {
        ?resource knora-base:lastModificationDate ?resourceLastModificationDate .
    }

[benjamingeer]

The WHERE clause binds variables that are used in the DELETE and INSERT clauses. If a variable is unbound, any statement in the DELETE clause that uses that variable will have no effect. We have to delete any existing lastModificationDate before we can insert a new one. Therefore we need to know the current lastModificationDate so it can be deleted.