Added start_activity function and tests

I think a hash is cleaner.
```ruby
# example usage
start_activity app_package: 'io.appium.android.apis', app_activity: ''.accessibility.AccessibilityNodeProviderActivity''
```


```ruby
# implementation
def start_activity(opts)
  app_package = opts[:app_package]
  raise 'app_package is required' unless app_package
  app_activity = opts[:app_activity]
  raise 'app_activity is required' unless opts[:app_activity]
  app_wait_package  = opts.fetch(:app_wait_package, '')
  app_wait_activity = opts.fetch(:app_wait_activity, '')

  unknown_opts = opts.keys - [:app_package, :app_activity, :app_wait_package, :app_wait_activity]
  raise "Unknown options #{unknown_opts}" unless unknown_opts.empty?

  execute :start_activity, {}, { appPackage:     app_package,      appActivity: app_activity,
                                 appWaitPackage: app_wait_package, appWaitActivity: app_wait_activity }
end
```