Info/Documentation for static functions in p5.Vector

#346 
Adds angleBetween to documentation
Adds examples for most instance methods which have a static version
Refines description of add, sub, mult, and div to more accurately describe p5.Vector
Fixes minor syntax issue with main example

*The one part of p5.Vector I didn't really touch on was method chaining. As this functionality for the Vector class is new to p5 and not in processing, it feels like there needs to be a bit more discussion on how to properly present & introduce chaining across the libraries in p5.*