Add HTTP proxy support

[mdeeg]

HTTP proxy support for requests client via Bravado config

Example with HTTP proxy:

from bravado.client import SwaggerClient

config = {
# set HTTP proxies for requests
'proxies': {'http': 'http://127.0.0.1:8080'}
}

client = SwaggerClient.from_url('http://petstore.swagger.io/v2/swagger.json', config=config)
pet = client.pet.getPetById(petId=42).result()

[coveralls]

Coverage decreased (-0.4%) to 98.035% when pulling cabd9df on mdeeg:http-proxy-support into cbd756e on Yelp:master.

[coveralls]

Coverage decreased (-0.4%) to 98.035% when pulling cabd9df on mdeeg:http-proxy-support into cbd756e on Yelp:master.