Feature/r2.5.1 getting started

Staged at: http://stg-web101.sw.joyent.continuuity.net/cdap/2.5.1-r2.5.1_getting_started/en/developers-manual/index.html

Though there are a lot of files (259!) and changes (almost 12K!), we have reviewed them for the most part through the staging server and the wiki.

The files to look at are the ones outside of "cdap-docs": there is a change to the README and the examples that should be reviewed. The example changes are listed as the last five files of the change list:

cdap-examples/HelloWorld/src/.../examples/helloworld/HelloWorld.java
cdap-examples/Purchase/src/.../examples/purchase/PurchaseApp.java
cdap-examples/SparkKMeans/src/.../examples/sparkkmeans/SparkKMeansApp.java
cdap-examples/SparkPageRank/src/.../examples/sparkpagerank/SparkPageRankApp.java
cdap-examples/WordCount/src/.../examples/wordcount/WordCount.java

Other files to review:
Building-blocks/streams (https://github.com/caskdata/cdap/pull/592/files#diff-162), staged at
http://stg-web101.sw.joyent.continuuity.net/cdap/2.5.1-r2.5.1_getting_started/en/developers-manual/building-blocks/streams.html