Some modernization & code cleanup

* Move the Preloader class into its own file + add $wgAutoloadClasses entry
* Coding style tweaks, as per MediaWiki's coding conventions
* Converted from PHP-based i18n to JSON i18n
* Version 1.3 with the appropriate changelog entries and whatnot
* Deprecated Preloader.php (but didn't fully remove it yet) in favor of extension.json -- MediaWiki 1.25 or newer is now required for this extension
* Added myself into authors
* Changed the Special:Version URL back to MediaWiki.org as it should be the canonical location for extension (and skin etc.) documentation; since this is an updated version of [[mw:Extension:Preloader]] it should be no problem to update that page to point to the correct repository etc.
* Tweak the README file to account for new MW requirements + extension loading method
* Fix $wgPreloaderSource so that it works again ( HT @legoktm )

extension.json

@@ -0,0 +1,31 @@
+{
+	"name": "Preloader",
+	"version": "1.3",
+	"author": [
+		"Rob Church",
+		"Troy Engel",
+		"Jack Phoenix"
+	],
+	"license-name": "MIT",
+	"url": "https://www.mediawiki.org/wiki/Extension:Preloader",
+	"descriptionmsg": "preloader-desc",
+	"type": "parserhook",
+	"config": {
+		"PreloaderSource": {
+			"0": "Template:Preload",
+			"_merge_strategy": "array_plus"
+		}
+	},
+	"MessagesDirs": {
+		"Preloader": [
+			"i18n"
+		]
+	},
+	"AutoloadClasses": {
+		"Preloader": "Preloader.class.php"
+	},
+	"Hooks": {
+		"EditFormPreloadText": "Preloader::mainHook"
+	},
+	"manifest_version": 1
+}