contextGroup element added to schema

This new element allows grouping of databaseChangeLog child elements
 under a single context(s). This is mainly useful for grouping
 multiple changeSets under a single context without the need of
 copy / pasting the context multiple times.

We are using liquibase in conjunction with Maven and an internal maven plugin to generate migration and rollback scripts for our DBA's to use during deployments. These scripts are tied to a certain release using the context attribute. We prefer to use multiple changeSets in order to maintain the perception of atomic migrations. However, it is tedious and error prone to copy / paste the appropriate context in order for the plugin to pull in the correct scripts. 

I wanted to contribute this tag back to the community as I can see others benefiting if they are using Liquibase in a similar setup.