some changes to vis-2

[oweitman]

• add section Developement and Debugging to readme
• add subscribe/unsubscribe command to the vis object, that widgets can subscribe/unsubscribe datapoint by themselve
• add sendTo command to the conn object, that widgets can communicate with adapters

[GermanBluefox]

Do you need subscribe/unsubscribe and sendto for vis-2 widgets or for vis-1?

[oweitman]

To make Vis-1 Widgets in vis-2 compatible

In vis1 I have access to the socket object there I can send commands to the server. In the vis object of vis-2 there is only a limited version of the socket object.

These new commands are convenient commands for that I used in my widgets.

To subscribe object ids I used in vis 1 the updateState function. This is also an additional issue,
Please have also a look for #438

[oweitman]

thanks (also for fixing the missing type definitions)