@XmlElement notation consistency

When I opened the PR #58 there was no `@XmlElement` annotation defined, but the `@XmlElement` annotation was introduced in #187, before #58 was merged.

Now the `@XmlElement` has two properties: `cdata` and `namespace` which have different notations in Yaml and Xml mappings.

I think this could be fixed in order to improve consistency. What do you think about?


## Yaml
### Current notation
```yml
JMS\Serializer\Tests\Fixtures\BlogPost:
    properties:
        author:
            type: JMS\Serializer\Tests\Fixtures\Author
            groups: [post]
            xml_namespace: http://www.w3.org/2005/Atom
            xml_element:
                cdata: false
```
### Proposed notations
#### Option 1
```yaml
JMS\Serializer\Tests\Fixtures\BlogPost:
    properties:
        author:
            type: JMS\Serializer\Tests\Fixtures\Author
            groups: [post]
            xml_namespace: http://www.w3.org/2005/Atom
            xml_cdata: false
```
#### Option 2
```yml
JMS\Serializer\Tests\Fixtures\BlogPost:
    properties:
        author:
            type: JMS\Serializer\Tests\Fixtures\Author
            groups: [post]
            xml_element:
                namespace: http://www.w3.org/2005/Atom
                cdata: false
```


## XML
### Current notation
```xml
<serializer>
    <class name="JMS\Serializer\Tests\Fixtures\BlogPost" xml-root-name="blog-post">
        <property name="author" groups="post" type="JMS\Serializer\Tests\Fixtures\Author" xml-namespace="http://www.w3.org/2005/Atom">
            <xml-element cdata="false"/>
        </property>
    </class>
</serializer>
```
### Proposed notations
#### Option 1
```xml
<serializer>
    <class name="JMS\Serializer\Tests\Fixtures\BlogPost" xml-root-name="blog-post">
        <property name="author" groups="post" type="JMS\Serializer\Tests\Fixtures\Author" xml-namespace="http://www.w3.org/2005/Atom" xml-cdata="false"/>
    </class>
</serializer>
```
#### Option 2
```xml
<serializer>
    <class name="JMS\Serializer\Tests\Fixtures\BlogPost" xml-root-name="blog-post">
        <property name="author" groups="post" type="JMS\Serializer\Tests\Fixtures\Author">
            <xml-element cdata="false" namespace="http://www.w3.org/2005/Atom"/>
        </property>
    </class>
</serializer>
```