Add user item custom action.

Fixing the following with missing feature.
```ruby
client.record_action_on_item("create", iid)
# PredictionIO::Client::U2IActionNotCreatedError: {"errors":[{"field":"pio_action","message":"Custom action is not supported yet."}]}
#	from /Users/oldpi/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/predictionio-0.7.0/lib/predictionio/client.rb:642:in `record_action_on_item'
#	from (irb):69
#	from /Users/xxx/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.2.11/lib/rails/commands/console.rb:47:in `start'
#	from /Users/xxx/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.2.11/lib/rails/commands/console.rb:8:in `start'
#	from /Users/xxx/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.2.11/lib/rails/commands.rb:41:in `<top (required)>'
#	from script/rails:6:in `require'
#	from script/rails:6:in `<main>'
```