[SPARK-915] Separate the admin and user scripts (sbin/bin) and tidy up some floating files

This PR aims at separating the scripts used by platform administrators and those used by application developers. Reasons as described in SPARK-915 (https://spark-project.atlassian.net/browse/SPARK-915). This PR is part of SPARK-915 but may not be all. It just contains the minimum changes required by our customer's. Specifically, the changes made by this PR are: 
1) all admin and advanced/internal scripts are gathered into sbin folder and related paths are adjusted
2) all users scripts are gathered into bin folder and related paths are adjusted
3) added bin/spark to let users run their own spark application (to run an app, users need to firstly add app jars into SPARK_CLASSPATH and then use command "bin/spark <main class> ..." to run their application. A new env var APP_MEM is used to set JVM heap size of the app driver process instead of SPARK_MEM)
4) put the floating data files (kmeans_data.txt, lr_data.txt, pagerank_data.txt into the "data" folder)