1. Re-implementation of Lynn C. Rees' pull
request (https://github.com/freeplane/freeplane/pull/20):

 - Always add <freeplaneuserdir>/lib and all jars therein to all
   script's classpath.

 - Add-ons have a new 'lib' node which may contain jars or class
   files (not nested).

 - Lib files are installed with the add-on.

2. Additions and changes compared to the initial pull request:

 - Install add-on scripts to a new "private add-on directory"
   <freeplaneuserdir>/addons/<addonname>/scripts

 - Install add-on libs to <freeplaneuserdir>/addons/<addonname>/lib

 - Omitted other non-related changes, especially the fix for the
   'deinstall' spelling error

 - Enhanced the add-on related 'devtools' scripts, especially to
   support the new add-on lib: Better checks, more automation.

Possible enhancement: The add-on libs are added to the global
script classpath, not only to the classpath of the scripts to
which the libs belong to.