v0.3.0

This is the third rework of Nene (see #17), this time, less breaking than the previous rework, but some changes are hopefully for the better, this is the changelog:

• License changed from MIT to Zlib, just as SDL2.
• Now nene uses the singleton pattern, which means you can use Nene.instance() function any type to get the nene instance, instead of passing it by parameter.
• All modules uses the newer module system of nelua, just as Lua, you require the modules as local Nene = require 'nene'
• Removed core.nelua file, now, just requiring "nene" will return the main nene module only, instead of requiring all modules as before
• The SDL's wrappers are removed, now nene just calls and handle SDL directly.
• SDL libraries updated to latest ones provided by nelua-decl.
• All modules that wraps a SDL type will contain a get_raw method, which returns the wrapped value (with a nilptr check), for instance, nene uses the term "raw" to refer to any wrapped type, especially the SDL ones.
• The nene's math module is now split between various modules like nene.math.vec2, one module for each type, however, the general math module is still provided and is not considered deprecated.
• Now it's possible to use any arithmetic type for 2D vectors and rectangles using Vec2n and Rectn types, for instance, Vec2, Rect and Rectf are respectively Vec2n(number), Rectn(integer) and Rectn(number).
• As the math module, the Audio module got split between Sound and Music modules, while the Audio wraps both.
• All "destroyable" types implements the __close metamethod, including the main nene module.
• Some font metrics are added to Font module.
• Text rendering on TextTexture module it's still solid by default, but now it's possible to use blended render mode, which is much better for non-pixel-art fonts.
• Previously some functions that returned an value, ok_status now returns ok_status, value, this is done to enforce if the function run successfully.
• All the examples and documentation updated