added SOCKS[45] proxy support

[mumumu]

Closes #177

added SOCKS[45] proxy support and its client impl, of course including tests with socks[45] server!

Usage is the following.

require 'httpclient'

#  ssh -fN -D 9999 remote_server
clnt = HTTPClient.new('socks4://localhost:9999')

#clnt = HTTPClient.new('socks5://localhost:9999')
#clnt = HTTPClient.new('socks5://username:password@localhost:9999')

#ENV['SOCKS_PROXY'] = 'socks5://localhost:9999'
#clnt = HTTPClient.new

target = 'http://www.example.com/'
puts clnt.get(target).content

target = 'https://www.google.co.jp/'
puts clnt.get(target).content

[carsonreinke]

socks_socket could potentially be nil, probably need to raise above. Also var is defined within if condition, but that might be an acceptable style thing

[ArtsiomVahin]

Would suggest changing this line to elsif getenv('http_proxy') . As it now stands, setting "HTTP_PROXY" does not appear to work anymore, only works with "http_proxy"