Added display attribute to AbstractLiquibaseTask.
Using exec instead of passthru to run liquibase.
Added checkreturn attribute to AbstractLiquibaseTask.
Added passhtru attribute to AbstractLiquibaseTask.
It's true by default so the behavior of Liquibase tasks doesn't change.
Added outputProperty to AbstractLiquibaseTask.
Made members protected instead of private.
Added nested parameters and substitution properties to AbstractLiquibaseTask.

Usage is:
<liquibase-update ...>
  <parameter name="logLevel" value="info" />
  <property name="idtype" value="INT" />
</liquibase-update>

Which results in:
liquibase ... --logLevel='info' update -Didtype='INT'
Added a generic LiquibaseTask.
With nested parameters in AbstractLiquibaseTask, all we need is the command.
Removed parameter $lbprops as it isn't used anymore.
$lbparams have to be to the right of $lbcommand for command parameters to work.
Added documentation for the liquibase tasks.
Merge pull request #247 from HuffAndPuff/master

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