Json schema (#4369)

* Added empty generator for json schema (idl_gen_json_schema.cpp)
#4360

* JsonSchemaGenerator: output of tables implemented

current problems:
- typenames are not correct
- array types need to be deduced

#4360

* JsonSchemaGenerator: Corrected generation of typenames
Current problems: enum types not written correctly
#4360

* JsonSchemaGenerator: Added generation of enum types
#4360

* idl_gen_json_schema.cpp: Write required properties to schema
#4360

* idl_gen_json_schema.cpp: Export Types including namespace
#4360

* idl_gen_json_schema.cpp: Fixed Json format
#4360

* idl_gen_json_schema.cpp: Formatted according to google code style
#4360

* Checked in monster_test.bfbs with changes from master

* Added idl_gen_json_schema.cpp in CMakeLists.txt

* generate_code.bat: Added generation of json schema

* Added todo.md

* generate_code.sh: Added generation of json schema

* Addressed some review issues
- removed command line arg -S
- removed new lines
- fixed codestyle in template functions
- removed usage of stringstream
- idented json schema
#4360

* removed auto in idl_gen_json_schema.cpp

* idl_gen_json_schema.cpp: changed iterator declarations to auto
#4360

* deleted todo.md

* idl_gen_json_schema.cpp: Removed keyword "override" so that vs2010 can compile

* idl_gen_json_schema.cpp: switch statement in GenType handeles all enum-members

* idl_gen_json_schema.cpp: Removed cerr output

* idl_gen_json_schema.cpp: Avoid vector copying

* idl_gen_json_schema.cpp: Fixed identation of json schema output

* idl_gen_json_schema.cpp: Do not output empty descriptions
diff --git a/CMakeLists.txt b/CMakeLists.txt
index d3c6a7a..53d453b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -50,6 +50,7 @@
   src/idl_gen_python.cpp
   src/idl_gen_fbs.cpp
   src/idl_gen_grpc.cpp
+  src/idl_gen_json_schema.cpp
   src/flatc.cpp
   src/flatc_main.cpp
   grpc/src/compiler/schema_interface.h