Updated LICENSE and LICENSE headers

This updates the root POM to use the file LICENSE_HEADER.txt for the license headers. It also adds that file, taken from the Apache licensing tutorial. Also, LICENSE.txt is updated accordingly as well.

All other changes stem from running `mvn license:format` and are purely automatic.

This addresses [REEF-26] (https://issues.apache.org/jira/browse/REEF-26)

This closes #9
Change HttpServerShellCmdHandler to call the Driver's method directly
Remove synchronized block in EvaluatorAllocatedHandler which is not effective
Replace the use of + operator in String to use StringBuilder and append operation
Merge branch 'master' into ys-impl-scheduler-example
Add LICENSE header
Remove CallbackHandler in HttpServerShellCmdHandler which is not in use