New feature: dynamic include elements

It seems obvious and natural, but it was challenging for me to add dynamic includes implementation requested by one of Scriptella users. Here is an example of dynamic includes.

<query connection-id="db">
    SELECT version FROM schema_info
	<include href="${version}.sql"/>

This feature is especially useful for building Rails-like migrations. Later me or Mike will post a short how-to on a simplified migrations in Scriptella ETL.