fix issue #215: lager.app env overwrites default handlers

the default handlers defined at lager.app overwrites handlers defined previously by application:set_env(lager, handlers, ...) if lager application is started application:start(lager)
Merge pull request #216 from fogfish/master

fix issue #215: lager.app env overwrites default handlers
Merge branch 'master' of https://github.com/basho/lager