Update for Godot 3.1.x

[TGRCdev]

The goal of this PR is to make GDSQLite compile and function for Godot 3.1.x on Windows, OSX and Linux.

This pull request makes a number of changes:

• Updates the SConstruct file to input the correct godot_cpp library
• Removes the old binaries from the demo folder
• Updates godot_cpp submodule to the 3.1 tag
• Cleans up some of the demo project
• Adds the OSX dynamic library to the GDNative library

I have tested this commit as follows:

• Windows
  • Compiled for 32 and 64 bit on Windows 10 Home 64-bit
  • Demo ran with both 32 and 64 bit versions
• OSX
  • Compiled for 64 bit on OS X Yosemite 10.10.5
    • (32 bit support was deprecated with macOS High Sierra)
  • Demo ran with the 64 bit version
• Linux
  • Compiled for 32 and 64 bit on Debian 4.9.0 64-bit virtual machine
  • Demo ran on 64 bit version, could not get 32 bit Godot working to test

Closes #30

[khairul169]

Much appreciated, thank you! 😁