Make XML serializer put each ManyToManyField object on its own line

[Tim Graham]

The XML serializer serializes many-to-many relations on a single line:

<field name="categories" rel="ManyToManyRel" to="serializers.category"><object pk="1"></object><object pk="2"></object></field>

which isn't very readable. I suggest this format instead:

<field name="categories" rel="ManyToManyRel" to="serializers.category">
  <object pk="1"></object>
  <object pk="2"></object>
</field>

Although not strictly required for this ticket, the first commit in my PR will remove fixed values in the XML serializer's indent() calls (e.g. self.indent(1)) which are unfriendly to "nested fields" like Django MongoDB Backend's EmbeddedModelField which I'd like to serialize like this:

<django-objects version="1.0">
  <object model="serialization_.book">
    <field name="name" type="CharField">Hamlet</field>
    <field name="author" type="EmbeddedModelField">
      <object model="serialization_.author">
        <field name="id" type="ObjectIdAutoField"><None></None></field>
        <field name="name" type="CharField">Shakespeare</field>
        <field name="age" type="IntegerField">55</field>
        <field name="address" type="EmbeddedModelField">
          <object model="serialization_.address">
            <field name="id" type="ObjectIdAutoField"><None></None></field>
            <field name="city" type="CharField">NYC</field>
            <field name="state" type="CharField">NY</field>
            <field name="zip_code" type="IntegerField"><None></None></field>
          </object>
        </field>
      </object>
     </field>
  </object>
</django-objects>

[Tim Graham]

​PR