Implement call timeouts on client

[ecorm]

Implement call timeouts, as per the advanced spec.

[ecorm]

Client requirements:

• HELLO.Details.roles.caller.features.call_timeout|bool := true
• HELLO.Details.roles.callee.features.call_timeout|bool := true
• CALL.Options.timeout|integer
• Receiving INTERRUPT messages (not sure about this)

It's not obvious what the message flow should be for timeouts, but a reasonable guess would be that the same as call cancellation, with the exception that the caller does not send a CANCEL message. I raised this ambiguity in https://github.com/tavendo/WAMP/issues/155.

The feature would require significant changes to the Client and Session classes. This feature should be implemented along with #21.

[ecorm]

The message flow is as expected. The requirement are such easier to meet for the caller, so this issue has been split up into #35 and #36.