Add base_url method

Please, add base_url method, a-la on Airborne.

Airborne
↓
```ruby
Airborne.configure do |config|
  config.base_url = 'http://example.com/api/v1'
  config.headers = { 'x-auth-token' => 'my_token' }
end
```

Unirest
↓
```ruby
Unirest.base_url = 'http://example.com/api'
# http://example.com/api/auth?user=foo&pass=bar
response = Unirest.get '/auth?user=foo&pass=bar' 
```