Add method to set the default HttpClient

[paivaric]

This is usefull to create Pooling Clients like:

PoolingClientConnectionManager conMan = new PoolingClientConnectionManager(SchemeRegistryFactory.createDefault());
conMan.setMaxTotal(200);
conMan.setDefaultMaxPerRoute(200);
HttpClient client = new DefaultHttpClient(conMan);
ClientFactory.setHttpClient(client);

[subnetmarco]

With commit 009bb77 this can be done using the following methods:

Unirest.setHttpClient(httpClient);
Unirest.setAsyncHttpClient(asyncHttpClient);