Check log multithread safeness

[seppo0010]

Fixes #92

[paegun]

@thinkingfish the general opinion about testing race conditions is generally considering only what can be done w/i the system under test and test. So the general opinion is leaving out the testing framework and/or compiler-provided tools. Since clang is used, if present, TSan ( http://clang.llvm.org/docs/ThreadSanitizer.html ) should be considered.

[paegun]

@thinkingfish I think the tsan documentation leads too much towards c++11, but clang (C) and clang++(C++) both have tsan integration. I've posted the following gist which demonstrates via a small example tsan in C: https://gist.github.com/785c726acf592ef91137bd90614e1ede

If I'm not understanding "doesn't support libc" properly, sorry. My experience w/ tsan in a C codebase has been limited to portability only to a few linux variants.

[paegun]

@thinkingfish I've updated the gist to include gcc 4.8 and the undocumented bits required to use tsan.

@seppo0010 sorry for the general thread testing commentary on this specific PR, which looks great.