Changed the order of function calls within CocosDenshion::SimpleAudioEngine::end()
to first detach the OpenAL source from the buffers so that sound buffers can be successfully deleted afterwards.

Additionally, improved the documentation within the header file a little bit.
Added to various functions a check whether an OpenAL source exists on which OpenAL functions can be called.
Playback of background music starts now with the previously set volume, independent of the OpenAL source state.
Added a delete on the SimpleSoundEngine object within CocosDenshionTest::onExit() so that alutExit() gets called to release allocated resources by OpenAL.
More best practices fixes

- class order fixes
  - static methods (creators) at the beginning
  - then constructors / destructors
  - then init
- more `const` fixes
  - adds some const getters
  - some getters have 2 versions: `const` and no-const version
- renamed CocosDenshion::sharedEngine -> getInstance()
Merge branch 'more_more_best_practices_fixes' of https://github.com/ricardoquesada/cocos2d-x into ricardoquesada-more_more_best_practices_fixes
issue #2407: SimpleAudioEngine::sharedEngine --> SimpleAudioEngine::getInstance().
issue #2407: more best practices fixes, more getter and const..
issue #2407: Minor fix in CCTMXLayer.h
Merge pull request #3197 from dumganhar/ricardoquesada-more_more_best_practices_fixes

Merge PR https://github.com/cocos2d/cocos2d-x/pull/3192
[AUTO] : updating submodule reference to latest autogenerated bindings
Merge pull request #3200 from CocosRobot/updategeneratedsubmodule_1374247442

[AUTO] : updating submodule reference to latest autogenerated bindings
Merge branch 'openal-deletebuffers' of https://github.com/rafaelx/cocos2d-x into rafaelx-openal-deletebuffers

Conflicts:
	CocosDenshion/include/SimpleAudioEngine.h
	samples/Cpp/TestCpp/Classes/CocosDenshionTest/CocosDenshionTest.cpp
closed #2415: [OpenAL] Adding one line to delete SimpleAudioEngine singleton in SimpleAudioEngine::end().
issue #2415: Formating codes in SimpleAudioEngineOpenAL.cpp.
Merge pull request #3203 from dumganhar/rafaelx-openal-deletebuffers

Merge PR https://github.com/cocos2d/cocos2d-x/pull/3147
Formatting codes with cocos2d-x code style in SimpleAudioEngineOpenAL.cpp.
Merge pull request #3204 from dumganhar/develop

Formatting codes with cocos2d-x code style in SimpleAudioEngineOpenAL.cpp.
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