fix checks for SDK and native builds
fix the fix for issue #635 as discussed at http://hub.jmonkeyengine.org/forum/topic/proposed-fix-for-issue-635/page/2 and 
http://hub.jmonkeyengine.org/forum/topic/wirebox-error
add copying of jme3-testdata.jar to SDK
add copying of test sources to SDK build
Update CONTRIBUTING.MD

Initial outline.
Rename CONTRIBUTING.MD to CONTRIBUTING.md

fixed stupid MD uppercase. GitHub doesn't recognise it as a Markdown file like that.
Update CONTRIBUTING.md

Typos and extra sections.
- add iOS to build
- add optional libraries to SDK
- remove old XML from sdk build file
delete library folders when cleaning SDK project
add method to find the android ndk build executable in a common way for all sub projects that need to build a native android lib
reorganize build of Android OpenAL Soft lib
Merge commit 'a0ccd9984c328dda80df3acffad6095e76878c52'
Merge commit 'f5c841aac54c09cb038d942407245f440f1d02d6'
add native android lib for loading png and gif files.  Does not premultiply.  Uses stb_image (http://www.nothings.org/stb_image.c)
Change AndroidAssetManager to use the native lib to load png and gif files
Change stb_image to be auto downloaded during build
change android native build scripts to not execute tasks at configuration time
Add method to check for existance of NDK
Remove Android Native project from build natives property check.