Allows to encode empty tables as arrays.

An option is added to encode empty Lua tables to arrays. The option is _encode_empty_table_as_array_ and is set like this:

~~~lua
   cjson.encode_empty_table_as_array("on")
   assert( cjson.encode{} == "[]" )
~~~~

By default, the option is off.

This is the only commit that needs to be taken into consideration:
https://github.com/ignacio/lua-cjson/commit/a6f950fa3b1f5d2b50a405cca5a74000dcf3dd16

The others correspond to other stuff.