Allow loading of local/self-signed CA certs

A small patch to the client to add an option to allow a new ca cert, chain or path to be added to the store, allowing local/self-signed certificates to be used.  Also add the default system CA cert path to the store.

(see http://www.ruby-doc.org/stdlib-2.0.0/libdoc/openssl/rdoc/OpenSSL/X509/Store.html for an explanation of these options).