process http request for perf data logged from LoggingScope. Allow to access driver log files through http request.

This is to address issue REEF-29 https://issues.apache.org/jira/i#browse/REEF-29?filter=-1

Author: Julia Wang
email: jwang98052@yahoo.com

- changing jettyHandler in HttpServerImpl into non-static
- use REEFFileNames for log file names
- process http request for perf data
- address review comments for format issues

Closes #15