Expand JSON Serialization to entire object

[mfine]

We need full JSON serialization of the SBP messages. Presently, just the SBP base message is being JSON serialized. Some binary fields will need to be base64 encoded.

[mfine]

1. Why? Presently, only the base SBP message is serialized in JSON - we need the full message format fully serialized, while adequately handling binary fields which need to be base64-encoded.
2. How? Expand the python templates to support to_json_dict and from_json_dict methods for each message, with an augmented yaml spec indicating which fields need base64-encoding.
3. Testing Add serialization/deserialization exercising for each message to the suite of tests.

[fnoble]

Do we need to specify which fields need to be base64 encoded or is it clear from specifying a u8 array vs a string?

[mfine]

There's been problems with the strings in the past, since they're actually 20-byte arrays that are null terminated - the extra bytes screw up the serialization. I'll try and see if there's an easy way to eliminate augmenting the specs - but not sure yet.

[mfine]

For posterity, here's the hack I did during the hackathon to find serialization issues:

+def todict(obj, classkey=None):
+    if isinstance(obj, dict):
+        data = {}
+        for (k, v) in obj.items():
+            data[k] = todict(v, classkey)
+        return data
+    elif hasattr(obj, "_ast"):
+        return todict(obj._ast())
+    elif hasattr(obj, "__iter__"):
+        return [todict(v, classkey) for v in obj]
+    elif hasattr(obj, "__dict__"):
+        if hasattr(obj, "payload"):
+            obj.payload = base64.standard_b64encode(obj.payload)
+        if hasattr(obj, "name"):
+            obj.name = base64.standard_b64encode(obj.name)
+        data = dict([(key, todict(value, classkey))
+            for key, value in obj.__dict__.iteritems() 
+            if not callable(value) and not key.startswith('_')])
+        if classkey is not None and hasattr(obj, "__class__"):
+            data[classkey] = obj.__class__.__name__
+        return data
+    else:
+        return obj

[mfine]

Completed. Note, did not have to augment the message yamls to identity areas that needed base64 encoding.