Version in log

Example contents of `base/version.hpp`:
```cpp
#pragma once

namespace principia {
namespace base {

char const kBuildDate[] = "2014-11-01T16:21:12.3692967Z";
char const kVersion[] =
    "57edf100d45d9f72fdc85cbc58041cdcdb488339";

}  // namespace base
}  // namespace principia
```