Replace explicit dependency on slf4j-log4j12 with slf4j-api

This commit fixes warnings when including grok in other frameworks
that use slf4j, such as Grails. For a description on the issue see
http://www.slf4j.org/codes.html#multiple_bindings.
Merge pull request #17 from pinguinkiste/slf4j-fix

Replace explicit dependency on slf4j-log4j12 with slf4j-api