[IZPACK-1144] - New button field

This is to address http://jira.codehaus.org/browse/IZPACK-1144
Button fields allow the user to optionally run a custom action or validator.
A use can for this could be for testing a connection to a service (ldap, webapp).

Allows you to specify the label with the id attribute from the <field> tags.
Allow you to specify the text on the button with the id attribute from the <spec> tags.
Allow you to specify a success message if the custom action/validator ran successfully with the successMsg attribute in the <spec> tags.
Allow you to specify messages available to your custom action/validator in <msg> tags nested under the <run> tags.
```
<field type="button" id="label">
    <spec id="button.text" successMsg="success.msg">
        <run class="com.you.button.Test" >
            <msg id="error.msg" name="error"/>
            <msg id="warning.msg" name="warn"/>
        </run>
    </spec>
</field>
```
For an example experience please see the references Jira issue above for instructions of building a sample installer.
Documentation will be updated if looks good.