Add Snapshots for story state (save/load)

[JBenda]

a snapshot contains all data which are not contained in the story and are
neccessary to restore the current state. Because of that a snapshot contains
a global and all corresponding runners.

• For loading a snapshot the same story file must be loaded
• functions must be bind again after loading a snapshot

further notes:

• add new snapshot_interface to implement for not trivial data types
• implement runner_create_snapshot and globals_create_snapshot
• implement snapshot interface for access from user
• implement writing snapshot to file and reading it from file
• add new_globals_from_snapshot and new_runner_from_snapshot for story
• updated wiki

[JBenda]

Was merged with #49