Added namespace support for Tire indices

This extension allows you to "namespace" index operations in Tire:

    Tire.configure do
      namespace("foo")
    end

All indices will be then prefixed by the namespace:

    Tire.index("bar").name
    => "foo-bar"

Closes #12
[#12] Refactored the "index namespace" support to work across the API

Previously, only the <Index#name> method reflected the namespace.

This patch overloads the <Index.new> method to define the `@name`
variable with the namespace included.

Fixes #12.