app/bind: name variables in the App interface

I didn't remember what that bool was, and I don't think others do.
app: implement GetEnv

Also added this method to the bind.App interface.

Related to #991.
app: add two new methods to the bind.App interface

AddInstance and RemoveInstance will be used to manipulate the
TSURU_SERVICES environment variable.

Related to #991.
testing: ensure that FakeApp satisfies the bind.App interface
service: fix tests
service: use testing.FakeApp in tests
service: properly add instances to TSURU_SERVICES in the bind call

Still need to remove the instance from the list in unbind.

Related to #991.