Implement native function pointer check, addr conversion and register, update documents (#185)

Modified WASM runtime API:
- wasm_runtime_module_malloc()
- wasm_runtime_lookup_function()
Introduced runtime API
- wasm_runtime_register_natives()

core/app-mgr/README.md

@@ -0,0 +1,10 @@
+WASM application management
+=======
+
+## structure
+
+
+
+
+
+<img src="../../doc/pics/wamr-arch.JPG" width="80%">