Adding some simple ssl verification precautions and config options.

This is just a rough improvement to make it overridable. I'm not sure how errors should be handled on non-existent files, should gitlab-shell abort? Not install? Either way this at least makes the certification path configurable and looks for some sane defaults.

This turned into a bit of a bigger change than anticipated. It also has initial support to start working on #36 .

Original issue is at gitlabhq/gitlabhq#3445 .

This pull only addresses the gitlab-shell side of things. Any https requests made inside the rails code also needs similar attention.