pry-bond is now compatible to pry/pry@193a229

- Pry::BondCompleter is now a class that is initialized with a Pry instance (to deal with pry/pry@193a229)
- Bond does not use Readline directly, anymore. Instead, Bond gets its input from Pry#complete
- But Bond uses still a global state. The local Pry instance is set before each completion call (using a Mutex to deal with parallel completion requests)