Release 2.6.0 doc fixes

Fixes these issues:
- https://issues.cask.co/browse/CDAP-803 Document memory requirement of CDAP app unit tests
- https://issues.cask.co/browse/CDAP-822 Docs have dropped mention of windows version of curl
- https://issues.cask.co/browse/CDAP-844 Hello World Typo: it shouldn't 'sing'
- https://issues.cask.co/browse/CDAP-845 Remove "element" from docs and replace with "program"

It replaces https://github.com/caskdata/cdap/pull/744, which addressed CDAP-811(Get rid of custom scripts in examples), and fixes issues with https://github.com/caskdata/cdap/pull/730, which addressed CDAP-808 (Updated docs for cdap-examples)

It removes some duplicate copies of the libexec/curl.exe located in various examples sub-directories, as they were just copies of the one supplied in the Standalone SDK.

The results of this are staged at:
http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT/en/examples-manual/examples/purchase.html

Important pages to review:
- Each of the example pages, linked from http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT/en/examples-manual/examples/index.html
- Testing ("Running Tests from an IDE") http://stg-web101.sw.joyent.continuuity.net/cdap/2.6.0-SNAPSHOT/en/developers-manual/testing/testing.html#testframework

Added MAVEN_OPTS to the test commands in BUILD.md.
Added.