i18n behavior - allow custom primary key name on i18n table

As default, the i18n behaviour mirrors the primary key of base table to i18n table. Changes in this PR allows to specify the name of created PK column in 18n table in schema through `<parameter name="i18n_pk_column" value="custom_pk" />`.

Use case - I am creating propel schema based on existing database structure, where stored localized texts suggests the use of i18n behavior. The structure can't be modified, because of legacy applications using it.

The simplified schema may looks following:

```xml
<table name="item">
  <column name="item_id" required="true" primaryKey="true" autoIncrement="true" type="integer" />
  <column name="text" type="varchar" />
  <behavior name="i18n">
    <parameter name="i18n_columns" value="text" />
    <parameter name="i18n_pk_column" value="custom_pk" />
  </behavior>
</table>

<table name="item_i18n">
  <column name="custom_pk" required="true" primaryKey="true" autoIncrement="false" type="integer" />
  <column name="locale" type="varchar" size="5" required="true" />
  <column name="text" type="varchar" />
  <foreign-key foreignTable="item">
    <reference local="custom_pk" foreign="id" />
  </foreign-key>
</table>
```

However without changes from this PR, the i18n table will get another column called `id` which will in turn become part of the primary key of this table.

I will be happy to prepare documentation for the new parameter, if this PR is going to be accepted eventually.