implementation of loading screen for sketches #317

I've implemented the loading screen functionality into the core.js. I've added code comments for the loading screen but was a bit unsure about the inline documentation. Since I did not add the loading screen functionality as a new function, the docs about inline documentation didn't seem as applicable as I didn't write a new function or method for the loading screen. The reason being is that it looks like logic needed early on in the library and located in the p5 constructor isn't broken out into separate functions — doing so seems as though it would go agains the current coding style of the library — at least in the core.js and the constructor.

I also included an example of the loading screen in the /examples directory. I referenced the cursor example and built a loading screen around that example. The new loading screen example is in examples/loadingscreen/index.html.

I was thinking that it would make sense to add documentation after the pull request was complete so we didn't have docs in the wiki further ahead than the current master branch. I'll update the wiki in the following days after the merge.

Let me know if there's anything that could be updated per the style guide, functionality etc.. If you wanted docs completed for new functionality within the constructor I'll write those up as well.

Let me know if you see any updates, or have suggestions to my solution.I could look into unit tests if you see appropriate as well. 

Cheers,