adding `--devices` option (mac only) to list attached video devices

* mac-only for now (outputs nothing on other platforms)
* some tidy-up in all capture classes (introduction of a common `executable_path` method)
* in a future PR, i'll handle this option on linux (maybe with something like [this](http://stackoverflow.com/questions/4290834/how-to-get-a-list-of-video-capture-devices-web-cameras-on-linux-ubuntu-c)
* README and CHANGLOG updated
* tests passing and :green_heart: 

after merging, we can probably close #183 and #174, and i'll update this [wiki page](https://github.com/mroth/lolcommits/wiki/Configure-Commit-Capturing) with info on the `--devices` option