Remove Underscore dependency

This was already done in #109 but [one call was left out](https://github.com/jeromegn/Backbone.localStorage/blob/master/backbone.localStorage.js#L58) and this caused underscore to be added again in #138.

I don't think it makes sense to drag it for one trivial call so I inlined `_.isObject` and removed the dependency yet again.

I also removed it from `dependencies` in `package.json`, #109 did not do it. It's now in `devDependencies` for the sake of the tests.