Implement the GnuTLS version of mailstream_low_ssl_get_certificate_chain()
Add a way for users to know the API current/revision/compatibility number
Add COMPATIBILITY for completeness
Use dynamic allocation for cert export
Same for mailstream_ssl_get_certificate()
Fix extra byte at end of DER certificates, that is unneeded.
Merge pull request #132 from colinleroy/master

GnuTLS implementation for mailstream_low_ssl_get_certificate_chain().