Add JSON checks

Use json-pointer module.

Syntax:

# Check that json has elements, without checking their value
json-has:
  - foo/bar/0
  - foo/bar/1
# Same, but check also their value
json-is:
  - foo/bar/0: qux
  - foo/bar/1: quz
# Exact correspondance of the json response
json-is-exactly: '{"foo":{"bar":["qux", "quz"]}}'