Logging refactor

Fixes #1448, #3275, #853, #3245

I've made the following changes:

* Documentation now instructs users to create a `logback.xml` file
* Play looks for log configuration files in the order
  `application-logger.xml`, `logger.xml`, `logback.xml` to maintain
  backwards comptaibility.
* Deleted `logback.xml`, Play never even used this.
* Play falls back to `logback-play-dev.xml` in dev mode, and
  `logback-play-default` in prod mode.
* Renamed `logger.xml` to `logback-play-default.xml` and
  `logback-play-dev.xml`
* Tuned the layouts - made the console logger print 10 lines of
  stacktrace instead of 5, since so often people report bugs and the
  first 5 lines of stack trace are not useful.  Also modified the file
  layout, it now logs messages all on one line except the exception, and
  doesn't put a double new line between messages.