remove composer dependency on composer-asset-plugin & use asset-packagist instead

This works the exact same, but without the requirement of a globally installed plugin
This _may_ require a "composer global remove composer-asset-plugin remove" to work correctly.

README.markdown

@@ -47,9 +47,8 @@ check out one of the versioned branches.
 1. Download shimmie via the "Download Zip" button on the [develop](https://github.com/shish/shimmie2/tree/develop) branch.
 2. Unzip shimmie into a folder on the web host
 3. Install [Composer](https://getcomposer.org/). (If you don't already have it)
-4. Run `composer global require "fxp/composer-asset-plugin:~1.1" --no-plugins`. (This is installed globally due to a known composer bug)
-5. Run `composer install` in the shimmie folder.
-6. Follow instructions noted in "Installation" starting from step 3.
+4. Run `composer install` in the shimmie folder.
+5. Follow instructions noted in "Installation" starting from step 3.
 
 ## Upgrade from 2.3.X
 

composer.json

@@ -4,6 +4,10 @@
 	"minimum-stability" : "dev",
 
 	"repositories" : [
+		{
+			"type": "composer",
+			"url": "https://asset-packagist.org"
+		},
 		{
 			"type" : "package",
 			"package" : {