hostname as a base param

In case you:
 - send anonymous hits (eg. via client_id=555),
 - AND use one ga tracking_id for more than one domain, eg. mysite.com and mysite.co.uk, you need to specify a hostname param for each hit you send.

Otherwise, in GA, the hit (no matter what kind of hit it is: event, transaction, pageview, etc) will be counted for each domain at the same time.

This commit fixes it by allowing to set a base param called 'hostname' which is then used for every hit we send.