Create an example of a custom extension point

[Nicklas Nordborg]

The manual ​http://base.thep.lu.se/chrome/site/latest/html/developer/extensions/extensionpoints.html mentions that it should be simple for an extension to define their own extension points. This has, however, not been tested in real life and there are some existing problems with this (see ​http://base.thep.lu.se/ticket/2041).

To make this feature easier to test I think the example code package should include a simple example of a custom extension point.

[Nicklas Nordborg]

(In [4238]) References #922: Create an example of a custom extension point

Adds the "net.sf.basedb.examples.extensions.custom-extension-point" extension point. The simple action, ExampleAction, is used only for listing all implementation of the extension point via the ListExampleActions servlet that is called from the menu entry Hellow world examples -> Example actions.

[Nicklas Nordborg]

(In [4239]) References #922: Create an example of a custom extension point

Added information about custom extension point to README.