Migrated and improved module to drupal 7

Hello everybody,

i've forked the module (https://github.com/georgwaechter/drush_migrate) and updated it to work on drupal 7.x. Additionally i improved and changed a few things:

- renamed module everywhere to drush_drake
- migrated functions to d7 (especially database calls)
- updated code to work with latest drush version correctly
- changed location of files to make it compatible with development workflows using drush make
- drake scripts are located within a "migrations" folder, which could reside in every module or installation profile
- removed features for creating scripts via GUI (in my opinion this should be a seperate module)

@the maintainer: What are your plans for the module in the future? And what do you think aboud the changes i made?

Best,

Georg