Added alias mapping option for entity namespaces

Add the alias for a namespace according to the mapping options:

``` php
"orm.em.options" => array(
"mappings" => array(
    array(
        "type" => "annotation",
        "namespace" => "Foo\Entity",
        "alias" => 'Foo',
        "path" => __DIR__."/../src/Foo/Entity",
        "use_simple_annotation_reader" => false
    )
)
```

Adjusted README & added the unit test for the option