changing structure so that the package can be installed with a standard procedure

Hi masa16,

thanks for writing this great package. I wanted to use it and hopefully make the installation procedure easier for everybody. I had to change the structure at least for the core and gen folders (I didn't touch the other extensions yet since I don't need them at the moment).

Now the package can be required from a Gemfile directly with

```ruby:
gem "narray", :git => "https://github.com/Enucatl/narray-devel.git"
```

and everything is generated and compiled correctly.

I hope this helps in making this new development package more popular.