added Migration::Helper module

This adds helpers to allow creating the search indexes more easily. Fixes #116

Warnings:
* Not a single line of automated tests included. If you are not going to write tests for this, please consider some manual testing before merging it into branch.
* Style might not be in-line with the rest of the gem code.
* Might not be in-line with mainstream gem. Was tested with the version of 6 months ago.