Mild cleanup / build fix

Removed missing readme.txt from Brimstone project: The Visual Studio project pointed to a readme.txt that wasn't in the Git repository. I removed the reference to keep the project clean.
Added a .gitignore to the repository to prevent lib/ and its contents being committed: You generally don't want to keep binaries in source control.
Converted brimstone/Exception.hpp and src/Exception.cpp to use macros: The macros automate the process of copying the declaration and definition. Also, LuaException was basically a reimplementation of Exception, so I changed LuaException to derive from Exception.
Added a forward declaration to get WindowsWindow.hpp to compile: Apparently, 'Key' did not exist in the scope of WindowsWindow.hpp. Easy fix.