0.5.1 Bugfix and maintenance release

Release 0.5.1 fixes a buffer overrun in the JSON printer and improves the portable libraries <stdalign.h> compatibility with C++ and the embedded newlib standard library. JSON printing and parsing has been made more consistent to help parse and print tables other than the schema root as seen in the test driver in test_json.c. The monster_test.fbs file has been reorganized to keep the Monster table more consistent with Googles flatc version and a minor schema namespace inconsistency has been resolved as a result. Explicit references to portable headers have been moved out of generated source. extern "C" C++ guards added around generated headers. 0.5.1 also cleaned up the low-level union interface so the terms { type, value } are used consistently over { type, member } and { types, members }.