Implementing array varibles

Adding mg_vprintf and mg_vprintf_data.

Fix default windows FD_SETSIZE, closes #506

https://support.microsoft.com/en-us/kb/111855 describes
that the user is expected to set the FD_SETSIZE macro
before including winsock2.h

Fix mjpg_streamer example

Inspect POST buffer in mg_get_var() only if first call of get_var() returns -1

Add a Gitter chat badge to README.md

In mg_get_var() the first call of get_var() inspects the variables of
the query string. If the requested variable is found but the destination
buffer is too small to hold the variable, return -2 right away. If it's
not found, make the second call of get_var() to inspect the POST buffer.

Reverting (socket < 0) checks to avoid Windows build failure.

Adding a check to avoid undefined behavior if socket is out of the bound...

safety belt : scan_directory() can return without setting arr to a valid value

OS2 and Android NDK fixes

Note that in Android NDK st_mtime is long unsigned int

Correctly handle _CRT_SECURE_NO_WARNINGS already being set in project se...