Add a Gitter chat badge to README.md

add elasticsearch 5.0.1 custom edition to skywalking-storage lib. use it as data index sevice (independence process).

# Conflicts:
#	skywalking-storage-center/skywalking-storage/src/test/java/StorageClient.java

add CONTRIBUTING.md

Fix some resources leak. Format log. Set jdk level of common lib to 1.6 . Fix some incorrect pom.xml .