Use slf4j as an abstraction for logging

The use of slf4j has several advantages:

• logger implementation can be exchanged at deploy time,
• at the moment log4j and apache commons logging are used, which results in different configurations.
• Bridge implementations allow slf4j to mime apache commons and log4j (and some others)
• thus resulting in streamlined Logging + configuration in one place. (even for foreign libs and their dependencies)

Logging dependencies are already growing see XWIKI-1839

More info at: http://www.slf4j.org/