Remove "LOG" Macro

The macro "LOG" should be removed because the name "LOG" is too common and conflicts with
some other products.
For example, current mongodb is not work with [google-glog](http://code.google.com/p/google-glog/).
It also uses "LOG" macro for logging.
Here is a sample code:

    #include <glog/logging.h>
    #include <client/dbclient.h> // <- undefs LOG macro
    
    int main() {
      LOG(ERROR) << "hoge"; // compile error
    }