Begin work on proper MyBB Application and Extension system

[xaoseric]

Added a class to properly register the MyBB instance, and creates base Extension class to be used by extensions.

[xaoseric]

@euantorano should be able to begin work on the installer soon, also this should make it easier to implement an api and separate interfaces for the forum, admin, and api

[euantorano]

Can you please fix the code style issues? You can ignore the JS one - I need to look at why it's checking JS...

[xaoseric]

Yes, will do that now

[katosdev]

@xaoseric @euantorano

[euantorano]

Some of my above points still hand, especially the image validation.