JSON: Add basic declarations for JSON output framework

All the JSON framework related interfaces are beginning
with 'json_'(for functions) or 'JSON_'(for macros or typedefs).

* defs.h: Add new typedefs for JSON output event and the corresponding
interface functions. Modify struct tcb to include JSON output hook
framework.
JSON: Add '-j' option to support JSON output

The output content is the same to the original strace
output, but the format is in a standard JSON format.

* strace.c(usage, init, alloctcb, droptcb): Add
support for '-j' option and call json_init()/json_free()
in alloctcb()/droptcb() to init/free FILE buffer in struct tcb.
* strace.1: Document -j option
JSON: Add the implementation for JSON output

* json.c: New File, Implement the JSON output framework.
* Makefile.am: Add json.c to compile.
Merge pull request #2 from zym0017d/json_base

Json base