Support trace propagation in httplib integration

[reyang]

With this change, the httplib will propagate context while sending requests.

Sample code:

from opencensus.trace import config_integration
from opencensus.trace.tracer import Tracer

config_integration.trace_integrations(['httplib'])
tracer = Tracer()

def main():
	with tracer.span(name = 'client'):
		response = requests.get('http://localhost:5000')
		print(response)

if __name__ == '__main__':
	main()

[reyang]

This is the same change as #220, created a new PR as the previous one was closed accidentally when I was trying to rebase.

[SergeyKanzhelev]

seems like intermittent failure in CI. Can you push empty commit to re-start the CI job please?

[bogdandrutu]

@SergeyKanzhelev I manually forced rebuild.