RAPI client: Switch to pycURL

Currently the RAPI client uses the urllib2 and httplib modules from
Python's standard library. They're used with pyOpenSSL in a very fragile
way, and there are known issues when receiving large responses from a RAPI
server.

By switching to PycURL we leverage the power and stability of the
widely-used curl library (libcurl). This brings us much more flexibility
than before, and timeouts were easily implemented (something that would
have involved a lot of work with the built-in modules).

There's one small drawback: Programs using libcurl have to call
curl_global_init(3) (available as pycurl.global_init) while exactly one
thread is running (e.g. before other threads) and are supposed to call
curl_global_cleanup(3) (available as pycurl.global_cleanup) upon exiting.
See the manpages for details. A decorator is provided to simplify this.

Unittests for the new code are provided, increasing the test coverage of
the RAPI client from 74% to 89%.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>