initial import from github.com/cihub/seelog

https://github.com/cihub/seelog/blob/5e83c34ad339ec49e50628f8b0fd86bfbfd78f68/writers_bufferedwriter.go
initial import from github.com/cihub/seelog

https://github.com/cihub/seelog/blob/64224fcfed140cb9a2fa4c51fd2dab48a6861ff6/writers_bufferedwriter_test.go
initial import from github.com/cihub/seelog

https://raw.githubusercontent.com/cihub/seelog/5e83c34ad339ec49e50628f8b0fd86bfbfd78f68/internals_byteverifiers_test.go
fix package name and make newBufferedWriter public -> NewBufferedWriter