lib/primary.js: Pass https.globalAgent.options.ca to https.get

Otherwise

  $ ./scripts/check_primary_support my.net /path/to/my/ca.pem

fails with:

  error: my.net is not a browserid primary: Error: SELF_SIGNED_CERT_IN_CHAIN

The user-specified certificate authority path was added in #3185
(3848755, Fix check_primary_support to read responses, 2013-04-03), but
because agent is set to false in the https.get call, the CA I'd specified was
not being used.

With this commit, I pass the globalAgent's ca through to https.get
[1], which wraps https.request [2], which accepts the ca option (among others)
from tls.connect [3].  Once it has the ca option, https.get verifies my
self-signed CA just fine (with Node v0.10.21).

I don't understand the httpProxy bit deciding between http.get and https.get,
or how key-verification works in that case.  It's possible that this patch
needs to be adjusted to handle that case, but http.get
[4], which wraps http.request [5] does not support the TLS options
(not much of a surprise ;).

I also don't understand why we set agent to false in the https.get call. 
0e39dc6 (Explicitly set rejectUnauthorized to false when using 
https.request(), 2013-08-08 [6]) mentions Node 0.8 needing both
"rejectUnauthorized: true" and "agent: false" to reject invalid certificated. 
Maybe we're far enough past Node 0.8 that we can remove the "agent: false"
part of this pair?  The "agent: false" bit dates back to the initial
primary.js addition in cc186ea (an abstraction around primary support checking
and a command line script to test primary support, 2011-12-19), so I'm not
clear on exactly what it's accomplishing.

There are also a bunch of other https.get calls in the codebase.  I'm not far
enough into my local Persona install to have a fully successful
check_primary_support, so it's possible that some of the remaining calls also
need the globalAgent CA data to get through check_primary_support with a
self-signed CA.

[1]: http://nodejs.org/api/https.html#https_https_get_options_callback
[2]: http://nodejs.org/api/https.html#https_https_request_options_callback
[3]: http://nodejs.org/api/tls.html#tls_tls_connect_options_callback
[4]: http://nodejs.org/api/http.html#http_http_get_options_callback
[5]: http://nodejs.org/api/http.html#http_http_request_options_callback
[6]: It really does say "set rejectUnauthorized to false" in the
    commit summary.  Oops :p.  The rest of the commit correctly sets
    rejectUnauthorized to true.