Some more functions implemented and a few fixes

The main thing I did here is implement a bunch of new attribute getters and setters for TTF_Font's.
I also changed some of the files in the testing folder to not use relative imports as it caused go get to spew complaints.
I also changed SDL_GetError to return a go error type as opposed to a string, to be more consistent with Go's method of dealing with errors (strings cant be nil!)