Windows: Fix dynamic library build with Visual Studio

Also correct a couple of calls to _to_json_string to check the return value.

This new method allows to also
get the length of the generated string.

Fix #165

Change-Id: Iea91404027f143ca3d29a4c58d7c07ae53556110
Signed-off-by: José Bollo <jose.bollo@iot.bzh>

To better distinguish between entry->k and entry->v being const within linkhash, but non-const outside, add lh_entry_v() and lh_entry_k() accessors.
Make lh_entry->k const.

Include stddef.h in json_object.h

Also, json_object_set_serializer is changed to respect the userdata
and user_delete parameters when to_string_func is NULL.

It is needed for size_t.

Add utility function for comparing json_objects

Performance improvements

This provides more control over some detail aspects, many
of which are performance related.

It's needed to squash a "variable 'val' set but not used" warning.

Apply compile warning fix to master branch

- Tabs are easier to read for tired eyes and editor adjustable

add bsearch for arrays

Remove trailing whitespaces

improve doc for json_object_to_json_string()

Arrays can already be sorted with json_object_array_sort() which uses
qsort() of the standard C library. This adds a counterpart using the
bsearch() from C.

* CVE-2013-6371: hash collision denial of service
* CVE-2013-6370: buffer overflow if size_t is larger than int

Add json_object_free_userdata() and json_object_userdata_to_json_string() too.

Conflicts:
	json_util.c