drush verify makefile

this is a command that we need for drupal.org. basically, we want the profile maintainers to run this before they try to use their .make file in a release, so they know drupal.org will build it successfully.

it parses the make file, validates it (with both regular and drupal.org specific validation), then downloads the update XML for core and each project to verify that the specified release actually exists.

the coding is done, but there is a bug or two and things aren't working exactly right...

also we need to make sure that 'drush convert makefile' is still working properly, since it was touched.