simplifies entry point and allows for command line execution

Took a quick look at your code and thought I'd clean a few things up. I'm not familiar with SketchUp's API so you might want to give this a good test before merging.

## Commit History

Replaced the `switch` logic in favor of a simple hash lookup table.

Gives actobotics_ChannelGenerator.rb executable permissions and
shebang (#!) interpreter for command line execution via:

    ./actobotics_ChannelGenerator.rb

There were also a few menu variables that had been redefined, I pulled out the
few that were being overwritten.

p.s. indentation/white-space ocd got the better of me.