Allow optional headers to the requests

The above change allows an extra "headers" config options. This is necessery for example for the FreeAgent API that requires a User-Agent header

You can then use it like this:

    var oauth2 = require('simple-oauth2')({
        clientID: 'SOMEID',
        clientSecret: 'SOMESECRET',
        site: exports.'SOMEURI',
        tokenPath: '/token_endpoint',
        authorizationPath: '/approve_app',
        headers: {'User-Agent' : 'require'}
    });

Note: ReadMe should reflect this

lib/core.js

@@ -46,6 +46,10 @@ module.exports = function(config) {
       options.headers = { 'Authorization': 'Basic ' + new Buffer(config.clientID + ':' + config.clientSecret).toString('base64') }
     else
       options.headers = {}
+
+    if (config.headers instanceof Object)
+      for(var header in config.headers)
+        options.headers[header]=config.headers[header];
 
     if (config.ca)
       options.ca = config.ca;