Allow to set custom host for receiving reports

This will allow to use different software (like errbit) to work with airbrake-js

* Add host option to XHR and JSONP reporters and generate urls based on this options
* Add setHost method to Client class, which sets new custom host
* Add fetching custom host from data-airbrake-host attribute
* Add tests
Merge pull request #108 from ronin/custom-host

Allow to set custom host for receiving reports