add XML Schema definition for the bundle configuration

This adds an XML Schema Definition file for the bundle configuration.

With this you'll be able to configure the bundle using XML like this:

```xml
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:translation-loader="https://github.com/maschmann/TranslationLoaderBundle/schema/dic"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
        http://symfony.com/schema/dic/services/services-1.0.xsd
        https://github.com/maschmann/TranslationLoaderBundle/schema/dic
        https://github.com/maschmann/TranslationLoaderBundle/schema/dic/translation-loader-1.0.xsd"
>
    <translation-loader:config driver="orm" history="false">
        <translation-loader:resource locale="fr">
            <translation-loader:domain>foo</translation-loader:domain>
            <translation-loader:domain>bar</translation-loader:domain>
        </translation-loader:resource>
        <translation-loader:resource locale="de">
            <translation-loader:domain>baz</translation-loader:domain>
        </translation-loader:resource>
        <translation-loader:resource locale="en" />

        <translation-loader:database entitiy-manager="default" />
    </translation-loader:config>
</container>
```

which is equivalent to the following YAML config:

```yml
asm_translation_loader:
    resources:
        fr: [ foo, bar ]
        de: baz
        en: ~
    driver: orm
    history: false
    database:
        entity_manager: default
```

The only thing I guess where we should find a solution is which URL to use for the Namespace-URL (using `https://github.com/maschmann/TranslationLoaderBundle/schema/dic/translation-loader-1.0.xsd` is imho no good idea).