examples/ubuntu: add HttpClient based on libevent

Based on the http-client sample in libevent source tree.
Still require hostname validation and ssl.

Bug: 24204632
Change-Id: Ifb34a3b8157aefc58e3f48ab241126b81187b934
Reviewed-on: https://weave-review.googlesource.com/1231
Reviewed-by: Johan Euphrosine <proppy@google.com>