Allow setting properties after instantiation

I find it more readable to assign properties/methods after instantiation,
like so:

```
$stub = new s();
$stub->foo = 'bar';
$stub->callMe = function() {};
```

This commit allows doing just that.

README.md

@@ -18,9 +18,13 @@ $stub = new Stub([
     'answer' => 42,
     'callMe' => function($a) { return $a; } # Anything which is a callable
 ]);
+$stub->foo = 'bar';
+$stub->myMethod = function() { return 50; };
 echo $stub->answer(); # => 42
 echo $stub->answer; # => 42
 echo $stub->callMe('maybe'); # => 'maybe'
+echo $stub->foo; # => 'bar'
+echo $stub->myMethod(); # => 50
 echo $stub->lol()->hey(); # => $stub
 
 // Different configurations

src/Nulpunkt/PhpStub/Stub.php

@@ -44,6 +44,11 @@ public function __get($name)
         }
         return null;
     }
+
+    public function __set($name, $value)
+    {
+        $this->methods[$name] = $value;
+    }
 
     public function __isset($property)
     {

test/Nulpunkt/PhpStub/StubTest.php

@@ -12,6 +12,20 @@ public function testThatAStubRespondToDefinedMethods()
         $this->assertSame(42, $stub->getId());
     }
 
+    public function testThatMethodsCanBeAssignedAfterInstantiation()
+    {
+        $stub = new Stub();
+        $stub->myMethod = function() { return 42; };
+        $this->assertSame(42, $stub->myMethod());
+    }
+
+    public function testThatPropertiesCanBeAssignedAfterInstantiation()
+    {
+        $stub = new Stub();
+        $stub->foo = 'bar';
+        $this->assertSame('bar', $stub->foo);
+    }
+
     public function testThatItHasAllProperties()
     {
         $stub = new Stub();