Option to pass a trusted server certificate when cloning

This fixes #414 by allowing the user to pass a trusted root certificate when cloning. This is necessary on OS X in sandboxed apps, as the libgit2/OpenSSL in a sandboxed app cannot for some reason access the system trust store for root certificates, and cloning with HTTPS fails.

Since I saw that all remote operations already have an `options` dictionary, I figured it makes the most sense to just add this option there. The downside is that the server certificate must be specified for all operations. Another option would be to have some kind of global options/configuration, but Objective-Git does not seem to have this notion, and I wasn’t sure how to go about adding it.

If we choose this API style to specify the cert, then I suppose a similar option should be added to all remote operations, but I figured I’d just start with one to kick this off.