Define Log structure
Move log.go into separate package
Merge branch 'master' of https://github.com/yunxing/xtree into yutong/log
rm log.go
Merge branch 'master' of https://github.com/yunxing/xtree
Implement Writer
Added writer_test
add record reader.
fail on test
Merge branch 'master' of https://github.com/yunxing/xtree into yutong/log
fix bug on reader_test.go, now passed test case
Updated test for writer
Moved costant from writer_test to writer
add const
pass backend test