Added fixing the xml config for most configuration options

[clairejarvi]

This allows things to be specified in xml as:

<rabbitmq:consumer>...</rabbitmq:consumer>
<rabbitmq:consumer>...</rabbitmq:consumer>

instead of:

<rabbitmq:consumers>...</rabbitmq:consumers>
<rabbitmq:consumers>...</rabbitmq:consumers>

It appears the the queues option in multiple_consumers might have the same issue but I am unsure how to fix it because fixXmlConfig() doesn't appear to affect it when I tried adding it.

Fixes #197.

[videlalvaro]

What's the purpose of fixXmlConfig() and what's the advantage of this PR, what is it fixing?

[clairejarvi]

This fixes the xml config differences according to Symfony's documentation:
http://symfony.com/doc/current/components/config/definition.html#normalization

For single entries:

<rabbitmq:consumers>
    <rabbitmq:consumer key="key" callback="callback">
        <rabbitmq:exchange_options name="name" type="fanout" />
    </rabbitmq:consumer>
</rabbitmq:consumers>

to

<rabbitmq:consumer key="key" callback="callback">
    <rabbitmq:exchange_options name="name" type="fanout" />
</rabbitmq:consumer>

For multiple entries:

<rabbitmq:consumers key="1" callback="callback">
    <rabbitmq:exchange_options name="name" type="fanout" />
</rabbitmq:consumers>
<rabbitmq:consumers key="2" callback="callback">
    <rabbitmq:exchange_options name="name" type="fanout" />
</rabbitmq:consumers>

to

<rabbitmq:consumer key="1" callback="callback">
    <rabbitmq:exchange_options name="name" type="fanout" />
</rabbitmq:consumer>
<rabbitmq:consumer key="2" callback="callback">
    <rabbitmq:exchange_options name="name" type="fanout" />
</rabbitmq:consumer>

[videlalvaro]

is this backwards compatible?

[clairejarvi]

fixXmlConfig() adds additional mapping capabilities but doesn't break the current behaviors. It'll end up mapping things in fixXmlConfig() like <consumer> to <consumers> during the tree building.