Added alias mapping option for entity namespaces

[marcojanssen]

Add the alias for a namespace according to the mapping options:

"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

[simensen]

Wow! Thanks for this.

One thing on the docs, though. Is it really just for annotations? It looks like the alias could live next to type and namespace options. Based on the code it looks like it should be able to be called for any of the mapping types. If it is open and should work for all, if you can move it around in the docs I'll merge this. :)

[marcojanssen]

Sorry about that, good point, moved it too it's own paragraph

[simensen]

@marcojanssen Thanks! Merged. If you can, please try out @dev to see how it looks and if it is good to go I'll try to tag a new stable tag sometime this week.

[marcojanssen]

@simensen It works just fine, works flawless. Using it in https://github.com/marcojanssen/silex-rest

stable tag will definetly be nice!