Changed schema for JSON test to be defined in a .proto file. (protoco…

…lbuffers#54)

* Changed schema for JSON test to be defined in a .proto file.

Before we had lots of code to build these schemas manually,
but this was verbose and made it difficult to add to the
schema easily.  Now we can just write a .proto file and
adding fields is easy.

To avoid making the tests depend on upbc (and thus Lua)
we check in the generated schema.

* Made protobuf-compiler a dependency of "make genfiles."

* For genfiles download recent protoc that can handle proto3.

* Only use new protoc for genfiles.

tests/json/test.proto

@@ -0,0 +1,42 @@
+syntax = "proto3";
+
+package upb.test.json;
+
+message TestMessage {
+  int32 optional_int32  = 1;
+  int64 optional_int64  = 2;
+  int32 optional_uint32  = 3;
+  int64 optional_uint64  = 4;
+  string optional_string  = 5;
+  bytes optional_bytes  = 6;
+  bool optional_bool  = 7;
+  SubMessage optional_msg  = 8;
+  MyEnum optional_enum = 9;
+
+  repeated int32 repeated_int32 = 11;
+  repeated int64 repeated_int64 = 12;
+  repeated uint32 repeated_uint32 = 13;
+  repeated uint64 repeated_uint64 = 14;
+  repeated string repeated_string = 15;
+  repeated bytes repeated_bytes = 16;
+  repeated bool repeated_bool = 17;
+  repeated SubMessage repeated_msg = 18;
+  repeated MyEnum repeated_enum = 19;
+
+  map<string, string>     map_string_string = 20;
+  map<int32,  string>     map_int32_string = 21;
+  map<bool,   string>     map_bool_string = 22;
+  map<string, int32>      map_string_int32 = 23;
+  map<string, bool>       map_string_bool = 24;
+  map<string, SubMessage> map_string_msg = 25;
+}
+
+message SubMessage {
+  int32 foo = 1;
+}
+
+enum MyEnum {
+  A = 0;
+  B = 1;
+  C = 2;
+}