root pom.xml: downgrade maven-jar-plugin to 2.3.2 because of manifest bug in m2e.
[ACT-1758] Add ability to flexibly configure the command interceptor chain by moving their configuration from the interceptors themselves to CommandConfig. Simplified the command-executor related functionality in implementations of ProcessEngineConfiguration. CommandExecutor split into seperate CommandExecutor and CommandInterceptor interfaces. CommandExecutor now has 2 execute methods, 1 with and 1 without the CommandConfig. CommandInterceptor only has the one with CommandConfig. CommandExecutorImpl renamed to CommandInvoker.