Added support for infinite and nan floats/doubles (#215)

- Added support for the custom double values from
   the protobuf json spec: "Infinity", "-Infinity", and "NaN"
- Added `infinite_floats` test data
- Updated Message.__eq__ to consider nan values
   equal
- Updated `test_message_json` and
   `test_binary_compatibility` to replace NaN float
   values in dictionaries before comparison
   (because two NaN values are not equal)

tests/inputs/float/float.json

@@ -0,0 +1,9 @@
+{
+    "positive": "Infinity",
+    "negative": "-Infinity",
+    "nan": "NaN",
+    "three": 3.0,
+    "threePointOneFour": 3.14,
+    "negThree": -3.0,
+    "negThreePointOneFour": -3.14
+  }