Simpler logger

Teach logger to work with std::ostringstream to simplify its use.
Instead of:
```C++
std::stringstream logmsg;
logmsg << "my favorite number is " << 40 + 2;
FairMQLogger::GetInstance()->Log(FairMQLogger::INFO, logmsg.str());
```
You now ~~can~~ __have to__ do:
```C++
LOG(INFO) << "my favorite number is: " << 40 + 2;
```
__std::endl__ is added by the logger on output.