add _writev again for batch write logs

[yss]

It is better to add _writev function for batch write logs.

The most important point is the performance that improve the speed of log.

but also there will cause some unexpected results, like

1. the size of log file may bigger that the maxSize when set maxSize config.
2. may cause more memory usage.

Also, I saw the deleted pull request at #25.
And I fixed the failed test cases.

[nomiddlename]

Hi, thanks for this. I totally agree that _writev should be implemented, but the travis CI build is failing because there are no tests which use this code. How do you know that it works? Have you done some performance tests so see if this improves things? Looking at the code, I can't see that this will make much difference to memory usage or performance - but maybe I am missing something.