Adding LogScope classes

To instrument reef code and measure the performance, this PR adds LogScope and LogScopeFactory classes. User can call LogScope with try-with-resources to measure the time elapsed in millis. 
LogScopeFactory supplies predefined methods, which return LogScope objects for the critical points/stages in Reef such as Driver start, Evaluator submit, etc . User can still create LogScope object directly for any piece of the code range. 
This is to resolve https://issues.apache.org/jira/i#browse/REEF-24?filter=12329580

I believe we can drop the `public` here as this class is only ever referenced in the current package.

This class should have checks whether the configured log level is even logged at this moment. If not, we can skip the (expensive) assembly of the strings.