Allow input customHeaders

Usage example:
var xero = new Xero(CONSUMER_KEY, CONSUMER_SECRET, RSA_PRIVATE_KEY, null, {'If-Modified-Since': '2014-07-10T00:00:00'});

GET Invoices ModifiedAfter a UTC timestamp
http://developer.xero.com/documentation/api/invoices/
The ModifiedAfter filter is actually an HTTP header: ‘If-Modified-Since‘. 
A UTC timestamp (yyyy-mm-ddThh:mm:ss) . Only invoices created or modified since this timestamp will be returned e.g. 2009-11-12T00:00:00
Merge pull request #4 from oitozero/master

Allow input customHeaders