Added a binary definition to be able to build the package easily.

I also refactored the code around a little.

## Package definition (package.json)

- added a binary definition, to launch a build of the package
- removed the unnecessary file extensions in the commands specified in `scripts` section, for simpler code

## Documentation

- documented the usage of the new command (binary) to launch the build of the package
- simplified the installation procedure which was too formal

## Binaries

- added the binary in charge of running the build of the package, for now it is completely equivalent to the usage of the build script

## Scripts

Refactored scripts for a better design:

- we keep actual scripts at the root, and put helpers/libraries in `node_modules`
- scripts all export an `exec` function, in case they want to be reused in some way, and they run this main function when they are directly invoked through Node.js

The grammar builder has also been cleaned, with clearer code, especially to manage paths, which is the most tricky part of it.

Closes #56