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
[REEF-3] Create basic Task scheduler as an example

  The current Task scheduler example only supports one evaluator
  at a time. This pull request addresses this issue to run multiple
  evaluators simultaneously and introduce a new REST interface to
  dynamically set the maximum number of evaluator.

  * The new API : `/reef-example-scheduler/v1/max-eval?num={num}`.
  * Quite a lot of code rearrange and refactoring are done.

JIRA:
  [REEF-3] https://issues.apache.org/jira/browse/REEF-3

Pull Request
  #23 https://github.com/apache/incubator-reef/pull/23

Closes #23