Update Slf4J to version 1.7.7

Update SLF4J from 1.5.8 to 1.7.7. Reasons:
* It's faster due use of ConcurrentHashMap internally.
* They introduced (after a loooong discussion) varargs for the placeholders. Instead of
```
Log.warn("Error trying to connect to remote server: " + hostname + "(DNS lookup: " + realHostname + ")", e);
```
we could write:
```
Log.warn("Error trying to connect to remote server: {}(DNS lookup: {})", hostname, realHostname, e);
```
It's more clear and a string concatination only happens when the string is logged.
In version 1.5.8 only _one_ (unboxed) placeholder is possible. Multiple parameters have to be boxed into an object-array, that's really annoying.

Some other improvements: http://www.slf4j.org/news.html

Do we need a Jira-Item for this patch?