Add Sandbox Support

[todofixthis]

Create a SandboxAdapter that enables developers to connect to remote nodes safely.

class SandboxAdapter(HttpAdapter):
  def configure(uri, token):
    # type: (Text, Text) -> SandboxAdapter
    ...

  def __init__(token, host, port=DEFAULT_PORT, path='/commands', poll_interval=15, sandbox_commands={'attachToTangle'}):
    # type: (Text, int, Text, int, Iterable[Text]) -> None
    ...

For non-sandbox commands:

1. Add header: Authorization: token {self.token}
2. Append the command name to the URI (e.g., https://sandbox.iotatoken.com/api/v1/commands/attachToTangle).
3. Return node response like normal.

For sandbox commands:

1. Add header: Authorization: token {self.token}
2. Append the command name to the URI (e.g., https://sandbox.iotatoken.com/api/v1/commands/attachToTangle).
3. Response will contain job info.
   https://github.com/iotaledger/documentation/blob/sandbox/source/index.html.md#attachtotangle
4. Poll every {self.interval} to see if the job has completed.
   https://github.com/iotaledger/documentation/blob/sandbox/source/index.html.md#jobs

[todofixthis]

Scheduled for release: 1.1.0