More features for Liquibase: Nested paremeters and some Exec-like attributes.

I needed the liquibase tasks a little bit more flexible for my own use and thought I'd share it.

AbstractLiquibaseTask (and thus all liquibase tasks) have some new attributes inspired by ExecTask and can also take nested parameters and substitution properties. Example of new functionality:
```xml
<!--
Old stuff like username and url excluded for brevity.
-->
<liquibase-update
    display='true'
    passthru='false'
    checkreturn='true'
    outputProperty='liquibase.update.output'
>
    <parameter name="logLevel" value="info" />        
    <property name="tablename" value="Person" />
</liquibase-update>
```

I also added a new liquibase task for when the liquibase command doesn't have it's own custom task yet.
```xml
<liquibase
    command='status'
    outputProperty='liquibase.status.output'
/>
```