Generator._generate_emailfield str.format refactor

_iter_for_choice -> random_str: Extract from Generator, drop the prefix 'val'
keyword, and drive iteration with a cheap shallow n-copy of the charset
Simplified Factory._verify_kwargs()
Simplify Factory._handle_build by making all kwargs iterables of dicts/None

Factory._handle_second_arg returns a tuple of kwargs and Factory._resolve_args
passes it along
Pass single model *arg cases to the main loop in Factory.build/create
factory.py: create and chain streams of instances instead of appending to
a list argument
Inlined Factory._build_instance, moved instance saving to 'decorator' generator
Moved common code in Factory.build/create into Factory._order_and_build/_deliver
Merge pull request #22 from lambdanaught/dev

Refactoring