added 'host' option for alternative DSTK endpoint

Allows alternate host for data science toolkit. Very useful for high volume geocoding with local [DSTK VM](http://www.datasciencetoolkit.org/developerdocs#setup).

Example:
```
var geocoderProvider = 'datasciencetoolkit';
var httpAdapter = 'http';
// optionnal
var extra = { host: '10.0.1.10' };

var geocoder = require('node-geocoder').getGeocoder(geocoderProvider, httpAdapter, extra);

geocoder.geocode('101 hyde st, san francisco, ca', function(err, res) {
    console.log(res);
});

geocoder.geocode('8.8.8.8', function(err, res) {
    console.log(res);
});
```