[split]  [util-app] add App registration

 Problem:

 It's sometimes useful to get a reference to the currently running App.
 You can hack your way their with a premain Hook, but I thought it would
 be easier to provide a simpler way.

 Solution:

 - Add an App companion object
   - Add a register(app: App) method, last write wins
   - Add a registered: Option[App] method to obtain the registed App, if any
 - in App's main method, call App.register

 Result:
 You can call App.registered to get the currently running App, if any

RB_ID=487357
[split] util-logging: factor out testing code into new package util-test

Problem

util-logging exported a dependency on scalatest. This causes broken
builds when used in Science, which is still on Scalatest 1.x.

Solution

Factor this out into its own package, and use selectively.

before% ./pants goal dependencies twitter-server/src/main/scala | grep scalatest
3rdparty:scalatest
org.scalatest:scalatest_2.10:2.2.2
before%

after% ./pants goal dependencies twitter-server/src/main/scala | grep scalatest
after%

RB_ID=482011