Implement Object.(get|set), make Object.set support callables, deprecate Context.(get|set|add_callable)

[qwenger]

Extended alternative to #81:

• Object.get and Object.set are implemented;
• Object.set supports callables, merging in the code from Context.add_callable;
• Context.get_global is implemented;
• Context.get, Context.set, Context.add_callable are reimplemented using Context.get_global and Object.get/Object.set and are marked as deprecated.

[qwenger]

The global part was implemented in #104 as a property.