Post process:
 - Adding a new method to manage post process
 - The post process are now independent of cameras.
 - It is possible to attach a single instance of a post process on multiple cameras or even camera several times.
 - The new method keeps compatibility with the old process.
- Fixing a bug on post processes indices.
- Fixing a bug on not reusable post process.
- Fixing a bug who could generate an error in PostProcess/babylon.postProcessManager.js
Merge branch 'master' of github.com:BabylonJS/Babylon.js into postProcess-merge-1.8.5

Conflicts:
	Babylon/Cameras/babylon.camera.js
Fixed detachPostProcess.
Fixed a bug from an automatic merge on babylon.camera.js
Merge pull request #132 from michael-korbas/postProcess-master

Post processes