Add an executable entry point

As of now, the only supported way of running gisdatigo is by using rails gisdatigo:run. This, when using rvm at least, means we have to add the gisdatigo gem to the Gemfile in order to have the gisdatigo:run task available. I usually just fire up irb, import the Gisdatigo module, and call the runner manually, and it works well. By supplying an executable we won´t need to temporarily add this gem to the project we want to update while still having a friendly interface.

I can work on this, if there´s no conceptual problem or misreading of the project idea.

What do you think?