SystemTimer::timeout may take one or two arguments; unit tests included

Cf. the conversation at https://groups.google.com/forum/#!topic/ruby-amqp/wLEslFxNkoM , this pull request includes two changes to support ruby 1.8.7: 

bunny/lib/system_timer.rb now defines the timeout method to accept either one or two arguments to match the SystemTimer gem 

spec/unit/system_timer.rb (new file) confirms that the timeout method does not raise an error when called with one and with two arguments.