Add stack package.
Fix return type in test helper.
Add CallerStackHandler and use package log15/stack in CallerFileHandler and CallerFuncHandler.
Trim stack traces in LazyHandler.
Remove out dated benchmark.
Merge branch 'master' of https://github.com/inconshreveable/log15
Document algorithm for Call's '%+s' format and replace obsolete comment about Go issue 8835 workaround with proper docs.