Add methods to modify reflections

It would be pretty cool to be able to modify a reflection, for example:

``` php
$newProperty = ReflectionProperty::create('bar', \ReflectionProperty::IS_PRIVATE | \ReflectionProperty::IS_STATIC);
$reflectionClass->addProperty($newProperty);
```

This is a powerful feature when combined with the ability to export a reflection to PHP code (see #133).
