Fixes #306

Fix typo

Fixes #300

Always preserve insertion order of object items

OOM crash 

and increasing hashtable order

Fixes a crash observed when there is OOM in hashtable_do_rehash

Polish API documentation

Simplify snprintf and vsnprintf checking for Visual Studio

The object_to_key_iter() example is now formatted like
json_object_foreach() and json_object_foreach(). The edited descriptions were
redundant, and the removed headers didn't add useful information.

The function signatures use "json", not "root".

Patch by Blanca Elio Rosario.

Don't crash on deeply nested values by limiting parser recursion depth

The fix limits recursion depths when parsing arrays and objects.
The limit is configurable via the `JSON_PARSER_MAX_DEPTH` setting
within `jansson_config.h` and is set by default to 2048.

Update the RFC conformance document to note the limit; the RFC
allows limits to be set by the implementation so nothing has
actually changed w.r.t. conformance state.

Reported by Gustavo Grieco.

Fix proto in doc

Implemented json_pack() format specifiers 's?', 'o?', and 'O?'.

Fix libdir is wrong path when compile with cmake

Optimized functions

because JANSSON_INSTALL_LIB_DIR is set to /usr/local/lib after
foreach loop

Optimized json_equal, json_delete, json_copy, json_deep_copy
Replaced if to switch
Removed unnecessary check for NULL.