Adds a default message when anvil help is invoked with no more arguments

When invoking anvil help, it raises an error:
```ruby
bundle exec bin/anvil help

/Users/manu/anvil-core/vendor/bundle/gems/activesupport-4.1.1/lib/active_support/inflector/methods.rb:238:in `const_get': wrong constant name {}Task (NameError)
	from /Users/manu/anvil-core/vendor/bundle/gems/activesupport-4.1.1/lib/active_support/inflector/methods.rb:238:in `block in constantize'
	from /Users/manu/anvil-core/vendor/bundle/gems/activesupport-4.1.1/lib/active_support/inflector/methods.rb:236:in `each'
	from /Users/manu/anvil-core/vendor/bundle/gems/activesupport-4.1.1/lib/active_support/inflector/methods.rb:236:in `inject'
	from /Users/manu/anvil-core/vendor/bundle/gems/activesupport-4.1.1/lib/active_support/inflector/methods.rb:236:in `constantize'
	from /Users/manu/anvil-core/vendor/bundle/gems/activesupport-4.1.1/lib/active_support/core_ext/string/inflections.rb:66:in `constantize'
	from /Users/manu/anvil-core/lib/anvil/task/naming.rb:20:in `from_name'
	from /Users/manu/anvil-core/lib/tasks/help_task.rb:18:in `task'
	from /Users/manu/anvil-core/lib/anvil/task.rb:56:in `run_task'
	from /Users/manu/anvil-core/lib/anvil/task.rb:26:in `run'
	from /Users/manu/anvil-core/lib/anvil/cli.rb:25:in `run'
	from bin/anvil:6:in `<main>'
```
With this fix, it shows a default messsage:
```code
bundle exec bin/anvil help

Usage: anvil help TASK_NAME [options]

Help for the anvil tasks. Usage: anvil help TASK
```