And add an -ifNotExists option to turn on the permissive behaviour

Related to https://www.exploit-db.com/exploits/45506

even though we document that you should set this in
   http://www.h2database.com/html/advanced.html#remote_access
people turn on network access but don't turn on -ifExists.

this limit applies to both 32 and 64-bit VMs

In order to make the doc generation for #476 working I had to fix some
errors in existing files. These cover

 - lines which are too long
 - incorrect indentation
 - invalid markup
 - missing javadoc

* Custom data types handler

* Updated doc, changelog, dictionary. Fixed review comment.

* Extended the custom types handler interface

* Fixed test

* Fixed review comments

* Added note on ABI stability of the feature to docs