Set env vars

It seems ec2 ami tools has changed their requirements; it now needs certain environment variables set.
[From the readme](http://aws.amazon.com/developertools/368):
> Before running the utilities the EC2_HOME environment variable
needs to be set:

> `export EC2_HOME=<installation-dir>/ec2-ami-tools-X.X-XXXX`

> ...

> The utilities can also be added to the path:

> `export PATH=$PATH:${EC2_AMITOOL_HOME:-EC2_HOME}/bin`

I also changed the name of the cookbook to reflect the name of the repository; this plays nicer with Berkshelf and Librarian-Chef.

Finally I added Chefspec and some simple unit tests. Chefspec isn't yet mature enough to test everything in the recipe, though.

ec2-ami-tools also requires Ruby, but that seems tricky to add as a dependency because users may want to install ruby themselves through rvm or whatever; I'm still trying to figure out how to use `include_recipe` conditionally so that the ec2-ami-tools recipe can install ruby only if it's not already installed.