#4 JSeq should use a logging framework

[Henrik Arro]

Currently, all logging in JSeq is done using System.out and System.err. This is not very flexible, and it causes problems if someone wants to use JSeq embedded in another program.

We should switch to use a real logging framework. For flexibility, it may be a good idea to use Commons Logging -- it is then easy to use different underlying logging packages.

To reduce the number of dependencies on other packages, the default implementation should probably use java.util.logging as log implementation.