Make Selenium Sessions JSON serializable

[jwoertink]

I'm not too sure about this, but I wanted to at least abstract out a discussion from #48 so people can at least chime in with their thoughts.

On the one hand, I can see how that may be useful to store in the DB, but on the other hand, what if someone wanted to serialize with YAML, or XML? Do we then have to start allowing PRs to serialize everything? Maybe there's a way we can create some hook or whatever so if people want to serialize in their own way, they can without needing a patch or whatever.