Add option to specify cwd for "svn info" command

Thanks for sharing this, really useful.

Right now when I run `npm test` against the repo I get failures, and couldn't see how to get them running (Apologies if this should be obvious... I'm not that comfortable with Coffeescript.)... so I've made a suggestion in this pull request. Basically it just contains an embedded SVN repo in the test/fixtures directory and has a new option to specify the cwd for the spawned process.

Feel free to discuss with me (or alter it yourself) if this isn't how you see it working.

It's worth noting that I also made the spawn() failure condition warn rather than being fatal. In my company people use a combination of the SVN client and Git SVN to work against the Subversion server so this would cause the build to fail for those using Git SVN. I see this as a grey area because you might want to *know* about a failure, but having code fail because of your choice of SVN client seems wrong to me, hence the `warn`.

Anyway, let me know what you think. I'll understand if you want to take it in a different direction.