added option to allow for streaming large files

Adds an option to stream large files without loading them into memory
Only the parsed response equals nil, eg.

$ bundle console
Resolving dependencies...
2.1.2 :001 > HTTParty.get('http://www.google.com', {stream_body: true, format: 'html'}) {|c| c}
 => #<HTTParty::Response:0x10 parsed_response=nil, @response=#<Net::HTTPOK 200 OK readbody=true>, @headers={"date"=>["Tue, 04 Nov 2014 22:41:52 GMT"], "expires"=>["-1"], "cache-control"=>["private, max-age=0"], "content-type"=>["text/html; charset=ISO-8859-1"], "set-cookie"=>["PREF=ID=cfe0e1d7007d33d8:FF=0:TM=1415140911:LM=1415140912:S=J9Rvte1zJFMYBMUD; expires=Thu, 03-Nov-2016 22:41:52 GMT; path=/; domain=.google.com", "NID=67=XjCdfG0qsNwZ1pO1EZ9MM3UKHVISp32nSdM1YeDlh7Ed4lN7_AF5svN3T8Pi5mJbZZPBcHI1vrCOSyZZfOTNbRDNnaX2TXtjSlDv1FodMmZ09SNYtLZnFAH-kn4dDmhw; expires=Wed, 06-May-2015 22:41:52 GMT; path=/; domain=.google.com; HttpOnly"], "p3p"=>["CP=\"This is not a P3P policy! See http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=151657 for more info.\""], "server"=>["gws"], "x-xss-protection"=>["1; mode=block"], "x-frame-options"=>["SAMEORIGIN"], "alternate-protocol"=>["80:quic,p=0.01"], "connection"=>["close"]}>