Enable server certificate verification by default

When https endpoint is used, the following message was shown: "warning:
peer certificate won't be verified in this SSL session".
This is because Net::HTTP#verify_mode is nil.

Now, we verify server certificate by default. If you want to disable it,
set Flickraw.check_certificate to false.
Add FlickRaw.ca_file option

Secure endpoint connection fails if CA certificate file is old or
is not installed.

So, add FlickRaw.ca_file option to set the CA certificate file path
even if we doesn't have root access right.
Changed SSL configs to match Net::HTTP options.