Add :transaction_id as an option parameter of Controller#send_flow_mod_{add,delete,modify}

Current Ruby Controller allows OpenFlow messages such as BarrierRequest to be sent with user supplied transaction_ids, but it is not possible to specify transaction_id when sending FlowMod messages using 
`Controller#send_flow_mod_{add,delete,modify}` methods.

This patch adds `:transaction_id` to the `options` hash to let user specify transaction_id 
when using `Controller#send_flow_mod_{add,delete,modify}` methods.