first pass at custom event types

.../java/src/org/sleuthkit/datamodel/BlackboardArtifact.java

...java/src/org/sleuthkit/datamodel/BlackboardAttribute.java

...ngs/java/src/org/sleuthkit/datamodel/CustomEventType.java

...ngs/java/src/org/sleuthkit/datamodel/TimelineManager.java

.../java/src/org/sleuthkit/datamodel/timeline/EventType.java

...c/org/sleuthkit/datamodel/timeline/StandardEventType.java

tsk/auto/db_postgresql.cpp

tsk/auto/db_sqlite.cpp