Dutch language updated for 7.8.

Dutch language file updated for release 7.8.
Meervoud

Meervoud: bestanden
letters->tekens & z -> s

Kleine typos verbeterd.
[ADD] Storage: added performance test for BlockFileAccess implementations; added MemoryMappedBlockFileAccess implementation.
Merge branch 'java-nio' of git://github.com/dimitarp/basex into java-nio