[UWS,TAP] Allow the specification of a custom logger in the configuration file.

This commit resolves partially the issue #28

Ideally, there should be an implementation of UWSLog and TAPLog working with
Log4j and another for SLF4J (and eventually for other logging mechanism).
Additionally, an implementation storing log messages in database would be
interesting. All these ideas may be implemented in UWSLib and TAPLib in a
future version.