Add support for the System Reached Start Level trigger

[nelsonaponte]

As described in the standard documentation for the core rules, JRule should provide a rule trigger for the System Start Levels 40 and up.

Example
@JRuleName("StartLevelRule") @JRuleWhenSystemReachedStartLevel(40) public void execChangedToRule() { logInfo("||||| --> Executing rule MyRule"); }

Only the following values, as specified in the rules documentation, should be supported:
40 - Rules are loaded and parsed, both from db as well as dsl and script files.
50 - Rule engine has executed all "system started" rules and is active.
70 - User interface is up and running. (planned, not included yet)
80 - All things have been initialized. (planned, not included yet)
100 - Startup is fully complete.

It should be possible to combine multiple triggers.
Optional, only one JRuleWhenSystemReachedStartLevel trigger should be valid as it doesn't make much sense to have multiple triggers of this type.