make guard synchronous

  * Guard.start + Guard.stop basically form a "main" loop

  * no "preserved state"

  * "no interactor" means interactor in interruptible sleep() mode

  * give Pry some time to start

  * use Queue to respond to changes in main Thread

  * handle signals asynchronously
refactor Guard Pry commands
deprecate rubygems < 1.8.0
get specs working with mock verification
Merge pull request #595 from guard/synchronous_guard

Synchronous guard + instance_double verification + fix specs