Move spec files into folder and make them requireable

Make it possible to require factories from outside the gem. This does the following:

1. Add `spec` to the list of files in the gem, that way it'll continue to work if it's ever used as an actual gem, instead of a git dependency
2. Add `spec` to the load path
3. Move all specs into `excelsio_core` folder, so they are namespaced and can be required without conflicting with other stuff.