Export json with id and add 'from_json' func

[dgercho]

Changes:

1. Added the node identifier (not presented when exporting to json format).
2. Added the ability to load a json blob into a 'Tree' structure.
3. Update the constant json outputs as part of the 'test_shallow_paste'.

[dgercho]

@liamlundy @caesar0301
Hi, any updates regarding this PR?

[cmoliverio]

When is this PR going to be integrated? Been half a year and this addition is quite small, but IMO critical. to_json needs to exist with a from_json counterpart, even if 1.8.0 rolls into 2.0.0 IMO. There is even a duplicate PR (#105) , and another PR that wants to add the identifier to the to_json, #123, so people clearly want this. . . I understand those are relatively easy to implement but this seems like a slam dunk that hasn't been dunking. @liamlundy