Add :transaction_id as option parameter of send_flow_mod
Avoid calls to get_transaction_id(), get_cookie() when possible.
Fix else clause format
Merge pull request #308 from y-higuchi/add_txid_to_send_flow_mod

Add :transaction_id as an option parameter of Controller#send_flow_mod_{add,delete,modify}
Disable rcov with Ruby 1.9 and 2.0
Now Trema compiles with 1.8, 1.9, and 2.0 (refs #272).
Use 2.0.0 as default Ruby version.
Trema gem should install with 1.8.7 or greater...
Fix illformed ruby version requirement.
Trema supports 1.8.7 or higher.
Updated installation instructions.
Shorten the rake task list shown with `rake -T`.