When initialising the logger, I can optionally specify the local log destination

Previously, when setting the `:local` parameter to true, it assumed a default log device (standard Rails log when in a Rails context, otherwise STDOUT).

Commit 6b7ea0c adds the ability to specify the local log destination (file name or IO) instead.

Commit be9c7d8 ensures that the default local log is relative to Rails.root when in a Rails context.

This pull request also includes a basic set of tests (using minitest) to make it easier to submit a PR without breaking random s#%t ;-)