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.proto

@@ -0,0 +1,12 @@
+syntax = "proto3";
+
+// Some documentation about the Test message.
+message Test {
+    double positive = 1;
+    double negative = 2;
+    double nan = 3;
+    double three = 4;
+    double three_point_one_four = 5;
+    double neg_three = 6;
+    double neg_three_point_one_four = 7;
+}