Thoughtful ordering of members about javapoet

[eamonnmcmanus]

I would like to restate here my preference from #113 that all methods, static or otherwise, be emitted in the order in which their MethodWriters were created. I don't see a common pattern in Java code of putting static methods in a particular place. In fact I often see methods change back and forth between static and instance according as they need to reference this.

[swankjesse]

For 3.0, I propose this ordering:

static fields
instance fields
constructors
methods
types

Within each group, the caller's declaration order is honored. We can make this fancier as necessary.