Migrate to using named loggers

Using named loggers allows client applications to have a fine grained control over the library logs. An example could be logging all sent emails for tracking purpose without setting all the application's logs at DEBUG which can be extremely verbose.

This commit names all the loggers used throughout securesocial, adds loggers to the sample app to demonstrate that there are no conflicts in defining a logger variable with the same name in the application code. Finally it changes the sample applications configuration to show how to set the log level for the securesocial loggers.
Remove unnecessary log level checks

The play.api.Logger API uses call by name parameters for the message arguments. They will be evaluated at each point they are used not at the point they are called. More information is available in http://eed3si9n.com/scala-and-evaluation-strategy and in http://www.scala-lang.org/docu/files/ScalaReference.pdf section 4.6.1 By-Name Parameters ( this document is for scala 2.9 but the call by name mechanism has not been changed significantly between the 2 versions)
Document the logging configuration for securesocial
Merge pull request #374 from jeantil/logger

Improve logging behaviour