provide executable jar

metrics-db currently has (at least) six classes providing a main method, which are invoked using six corresponding ant tasks.

One solution for providing an executable jar:

Add a new class org.torproject.ernie.db.Main as the jar's main class adding the following functionality:

• calling java -jar metrics-db.jar will show a usage message and exit.
• calling java -jar metrics-db.jar <taskname> will run the main method of the class called in the current ant task of the same name, i.e., taskname is one of
  • bridgedescs
  • bridgepools
  • exitlists
  • relaydescs
  • torperf
  • updateindex

Finally, remove the then obsolete ant tasks from build.xml.

What is missing? Should a task be added or removed? Other thoughts?