clean up slf4j dependencies
 - tiles should declare slf4j-api as depedency to be included transitively since its code references it,
 - tiles should not declare at all any slf4j implementation dependency in compile or runtime scope,
 - include in all test scopes sl4j-simple
 - bump to slf4j 1.7.6

reference: http://www.slf4j.org/faq.html#maven2


git-svn-id: https://svn.apache.org/repos/asf/tiles/framework/branches/TILES_3_0_X@1583199 13f79535-47bb-0310-9956-ffa450edef68