Updates to test suite and gem organization

I wanted to open this PR to gather feedback on the changes made and look into improving the library more.

Wanted thoughts on the following:

* Moving to Minitest
* Moving `URI::URN::Generic` into its own file
* Updating the gemspec to reflect common ruby library formats
* Moved `BSDL` to `LICENSE` and updated the `COPYING` text to reflect those changes
* Making the `VERSION` a constant under `URI::URN::VERSION`

I'd also like to hear about what is on a TODO list and where things are heading.
Changing file name from `BSDL` to `LICENSE` is okay. But the name of license is "2-clause BSDL".