Add property name translation for PLYLoader

The property names in PLY files are given as strings, some
implementations may use different names for common properties.
This commit adds a translation mechanism based on a map,
which is specified via the constructor.

Example with no translation:
  loader = new THREE.PLYLoader();

Example with translation:
  loader = new THREE.PLYLoader({
	  		diffuse_red: 'red',
			diffuse_green: 'green',
			diffuse_blue: 'blue'
		});
Explicit method to set the property name mapping

The property name mapping can now be set by calling
setPropertyNameMapping, instead via the constructor.

Example:
	loader = new THREE.PLYLoader();
	loader.setPropertyNameMapping({
		diffuse_red: 'red',
		diffuse_green: 'green',
		diffuse_blue: 'blue'
	});
Updated documentation and usage.
Merge branch 'master' of https://github.com/svenpilz/three.js into dev

Conflicts:
	examples/js/loaders/PLYLoader.js