Add '--dry-run' parameter

[cordoval]

ref https://github.com/cordoval/gush/issues/17

I'm proposing to add --dry-run parameter, that would actually show git commands, that would change data in repository instead of executing them.

This would be especially useful while library isn't stable to prevent of accidentally removing something useful from the repo with which manager tools will be working.

[cordoval]

at first it thought dry-run was going to log the git commands only, seeing they are trivial i think it is not a good idea just to see output, the command already runs git and outputs the output of those commands so it actually is verbose already. The second suggestion with pedantic mode was to log curl or api client requests into the cli which is possible provided we use guzzle, and guzzle has this by default. Maybe that could be something this ticket can implement.
This comes free by one of amazon's top devs @jeremeamia in his blog post http://blogs.aws.amazon.com/php/post/Tx1W2JMJBQHBNRS/Wire-Logging-in-the-AWS-SDK-for-PHP

[cordoval]

this was implemented, just set GITHUB_DEBUG for the github adapter and that is it, if someone wants to wire that part up to the upper Gush classes, feel welcome but i think this is more for troubleshooting so it can be enabled at any time