[api] Added support for using Jitsu through a proxy server.

[AvianFlu]

@pkumar was having trouble using jitsu from his university's network, so @dominictarr and I looked into the situation.

This patch adds support for a proxy property in .jitsuconf, which should be set to the hostname of the proxy server to be used. If proxy is set, the request headers are adjusted accordingly.

[indexzero]

Looks good to me.

[pksunkara]

This is not working for me.

info:   Welcome to Nodejitsu
info:   It worked if it ends with Nodejitsu ok
info:   Executing command login
prompt: username: pkumar
prompt: password: 
{ method: 'GET',
  uri: 'http://localhost:10001/auth',
  headers: 
   { Authorization: 'Basic *******==',
     'Content-Type': 'application/json',
     host: 'http://api.nodejitsu.com' } }
error:  Unable to Authenticate as pkumar
error:  Jitsu requires you to connect to Nodejitsu's stack (api.nodejitsu.com)

even with replacing host with Host

[pksunkara]

Refer #107

[AvianFlu]

@pkumar is right: I only set the headers, his pull request in #107 uses the proxy option in request, which allows for proxy auth and other such goodies.