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.
See the pom.xml to see see how the test dependency is included? If that is the meaning, then maybe we should just show what to add to <dependencies>, since it's pretty much 3-4 lines of config.