Add Launchy to support multiple OS browser configs (#26)

As per [discussion](https://github.com/mwunsch/tumblr/issues/26) in #26, I've added Launchy support and replaced the `open` command with Launchy's public API method.

I had to update the Gemfile so that versions of `addressable` would be agreeable between Launchy and Webmock. Let me know what you think!