In particular, the `GAE_MODULE_NAME` environment variable takes
priority over the `serviceName` config item and the
`GAE_MODULE_VERSION` environment variable takes priority over
the `serviceVersion` config item.

In particular, if the config object supplied to the `start()`
method has a `debug` attribute then the value of that attribute
will be used as the config.  Otherwise, as long as the config
object is defined, it is assumed to directly contain the
config options.

The values of `serviceName` and `serviceVersion` were previously
determined in `lib/debugletapi.js` by determining if certain
environment variables are set.  Now that check is done in
`index.js`.

The unit tests have been updated so that the `start()` method
is called when requiring `index.js`.

If the GCLOUD_DEBUG_LOGLEVEL environment variable is set then
its value will be used as the log level instead of the value
specified in the supplied configuration file.  However, the
unit test had the value of GCLOUD_DEBUG_LOGLEVEL and the value
in the config file both set to the same value.  Thus, the test
wouldn't verify if the environment variable was actually taking
precedence.

The tests verify that the value of the `GAE_MODULE_NAME`
environment variable (if defined) takes precendence over the
config value of `serviceName`.

Similar checks are also done for the environment variable
`GAE_MODULE_VERSION` and the config option `serviceVersion`.

The test in `test-module.js` that tests that two calls to
`require()` return the same object was updated to call the
`start()` method on both requires.

That is, the config is not expected to contain a `debug`
attribute that contains the actual configuration options.
This change makes the debug agent consistent with the trace
agent.

In particular, tests were added to verify that the order of
priority from highest to lowest is
```
  env var -> config given to start() -> config file
```