Create "composer test" command in order to run phpunit tests

As the project already is installed via composer, this command
could save a few keystrokes!

Besides that, is just one command for all OS, because will not be
needed to change slashes to backslashes in Windows.

*X      (21 keystrokes): `./vendor/bin/phpunit`
Windows (21 keystrokes): `.\vendor\bin\phpunit`
New (14 keystrokes):     `composer test`

https://getcomposer.org/doc/articles/scripts.md#writing-custom-commands

CONTRIBUTING.md

@@ -187,6 +187,12 @@ You can test the project using the commands:
 $ vendor/bin/phpunit
 ```
 
+or
+
+```sh
+$ composer test
+```
+
 ### Windows
 
 You can test the project using the commands:
@@ -194,6 +200,12 @@ You can test the project using the commands:
 > vendor\bin\phpunit
 ```
 
+or
+
+```sh
+> composer test
+```
+
 No test should fail.
 
 You can tweak the PHPUnit's settings by copying `phpunit.xml.dist` to `phpunit.xml`

composer.json

@@ -45,5 +45,8 @@
         "branch-alias": {
             "dev-master": "1.0-dev"
         }
+    },
+    "scripts": {
+        "test": "./vendor/bin/phpunit"
     }
 }