Add support for libgit2 git_transport_certificate_check_cb

[phatblat]

It looks like the old way of providing libgit2 with certificate locations doesn't work without OpenSSL #499.

After a little bit of research, I think that git_transport_certificate_check_cb (part of git_remote_callbacks) needs to be exposed somehow so that applications can provide custom server trust validation. ~~This may be essential for sandboxed apps if the new SecureTransport support in libgit2 doesn't already tap into the root CA certs.~~

This callback was added in libgit2 0.22

Update: This callback appears to be unnecessary for normal certificate validation. It's only needed for providing a custom cert.

[jaanus]

Everything works as expected for me with the latest code f2e17cd8c43cc8d34ccaf61afb7f0701e6414a89 in a sandboxed OSX app on Yosemite.