INT-2893: fix chain -> router -> bean or script

* add XSD element for nested router type
* add tests on the matter
* polishing for RouterParserTests

JIRA: https://jira.springsource.org/browse/INT-2893

INT-2893: Polishing

INT-2893: after rebasing to 3.0

XSD changes reverted from 2.2 and moved to 3.0
INT-2893 Polishing

Factor out common elements; fixing missing documentation for
`expression` within a `router` within a `chain`.
INT-2893: Polishing according PR comments