Brand new to Alexa (got my first Echo yesterday afternoon), but after some hackery I have managed to code an Alexa skill that will change channels in the app by name or channel number via Alexa and also mute, pause, skip, etc. The way I did it is very quick and dirty... the dirty part meaning I've exposed the Channels API for 2 of my Apple TVs to the internet. I plan on improving that, of course, and have a few ideas about the best way to achieve it. I guess it took me 3 or 4 hours in total, with the first 2 hours figuring out the basics of Alexa Skill architecture and coding.
Just to say, it's really quick and easy to code Alexa Skills. The IDE is pretty basic, but also easy to use.