[PERSISTENCE] Model::Persistence uses "string" as the default mapping type
[PERSISTENCE] Model::Persistence returns true/false for #save and #destroy operations

Previously, the Tire::Model::Persistence models returned the object itself after calling
`save` and `destroy` methods, making it cumbersome or impossible to do common routines such as:

    @article = Article.new params[:article]

    if @article.save
      redirect_to @article, :notice => "Article was successfully created."
    else
      flash[:error] = "There was an error when saving the article, please try again."
      render :action => "new"
    end

The #save and #destroy methods now return `true` for successful operations,
`false` for unsuccesful ones, making it possible to intercept common
failures such as trying to index a document with date in incorrect format,
trying to index a document with unmapped property, etc.