methods to drivers.

- What was drivers are now "handlers" (e.g. trigger vs. napalm)
- Handlers are bound to NetDevice objects
- Drivers are now bound to handlers and include all the variable bits

  - name
  - prompt_pattern
  - startup_commands
  - commit_commands
  - delimiter (line-ending)

- Renamed all "handlers" to "dispatchers"
- All Trigger drivers now inherity from TriggerDriver (Wow!)
- Added a TriggerDriver for 'juniper'
- NetDevice object now has 3 new methods that interact w/ the
  dispatcher:

  - .open() -> Establishes a dispatcher (trigger or napalm) and connects
  - .close() -> Disconnects and tears down dispatcher
  - .dispatch() -> Dispatches a method on the driver

- NetDevice.connected returns the connected state from the driver's
  session
- The BaseDispatcher is greatly simplified and only really cares about
  how to get a driver, how to dispatch from the driver, and whether the
  driver is connected

  - Credentials are still stored on the dispatcher, but will soon be
    moved to be stored directly on the NetDevice (per-device creds)

- Device is still stored directly on the TriggerDriver instance, but
  that will also go away once we replace the twister functionality w/ a
  streamlined client factory.
- The NapalmDispatcher is currently hard-coded to dispatch in a reactor
  thread using Crochet, but may be modified to return a naked deferred
  instead.

* The idea behind the new endpoint code is so all sessions make use of a
single channel pty protocol. This reduces the amount of redundant code
needed in twister.py.
* New streamlined IosLike endpoint code that runs within the crochet
reactor thread.
* Replaces the existing endpoint code.
* Running commands over a multiplexed session is no longer supported.

- Update TriggerEndpointDispatcher.driver_connected() for twister3
- Creds are now directly stored on the TriggerDriver but are still taken
  in at
  the constructor as username, password. We might swap this out.
- Driver.has_error is a driver-level callback to check results for
  errors (Evolved from trigger.twister.has_{vendor}_error() functions.

  - Added .has_error methods to IOS-like and Juniper drivers.

- Driver.delimiter and Driver.has_error are now sent to the client
  factory.

- Moved keyexchange stuff from ClientTransport.connectionMade to custom
  CiscoClientTransport which is now provided by the Cisco driver
- Transport_class can now optionally be sent to twister3.connect() and
  to twister3.ClientFactory

- Trying to make Commando2 work / twister3
- Still "some kind" of issue w/ calling _send_next (try running 'show
  ntp status' on a Juniper device)

- Apparently NetDevice.open() *does* block, such as if a device is
  unreachable. We need to make this not block!