Separate generator from the Apipie

The code for generating the bindings is a part of this gem now. The generator needs a running Foreman instance to load the apidoc.json.

Usage: `bin/generate.rb -h`:

    bin/generate.rb -h
    Script for generating API bindings for Foreman API from Apipie docs.
        -u, --url FOREMAN_APIDOC_URL     By default http://localhost:3000/apidoc
        -h, --help

The advantages are being more flexible on Foreman-specific changes (than having a common generator in the apipie), no dependencies on the apipie itself, less code generation gemplates (only resource files are generated now, everything else is just an ordinary ruby files).