Add "pebble" mode

[gvc]

Closes #29.

[coveralls]

Coverage remained the same when pulling 16fde1a on gvc:add_pebble_object into 0869d4a on avdi:master.

[avdi]

This makes me happy!

2 things, if you don't mind:

1. Make it into a command object instead of method on the builder.
2. I'm not a big fan of putting expectations on Kernel in tests. I'm also not sure why we need p here instead of puts, since we don't need to inspect the interpolated string. Can you update this to either a) capture stdout and test the captured tests; or b) optionally dependency-inject an "out" role and use that role to do the output? If you're not clear on how to do these I can take a crack at it.

Thanks!

[gvc]

Great @avdi, thank you for your input. I'll try and tackle these questions this week. If I get stuck somewhere I'll let you know!

[coveralls]

Coverage remained the same when pulling 8b615cb on gvc:add_pebble_object into 0869d4a on avdi:master.

[coveralls]

Coverage remained the same when pulling b2c68a0 on gvc:add_pebble_object into 0869d4a on avdi:master.

[coveralls]

Coverage remained the same when pulling 6ec840a on gvc:add_pebble_object into 0869d4a on avdi:master.

[julianalucena]

Hey, @avdi. Could you look at it again, please?

[avdi]

Rockin'!