Toggle fullscreen with "return"
Move enableTesting flag into conf.lua

Previously located in test.lua, but I felt it was more of a
configuration option than a variable to be set at the beginning of a
source file.
Mock out Desmond class
Move all imports into imports.lua

Write some documentation for the decision.
Slightly revise imports.lua

Testing is handled in imports.lua now, and main.lua doesn't call require
until partway into love.load()
Export desmond object in main.lua