Exact target gem

* gem will manage exact target subscriptions
* the only two method being use is edit_subscriber,
find_subscriber.
* Added test for the response on each method case success and
failure.
* TODO:
  * add test for xml template constructor
  * add test for add_subscriber
Add simple cov dot file
change data for test fixture
Add add_subscriber test coverage
descrase minimum coverage
Merge pull request #1 from amaabca/add-exact-target

Exact target gem