v0.9.0

1. code refactoring and doc enhancement
2. change FormBinder.bind (!!!WARNING: broken change for normal developers),

from

// bind mappings to data,
// consume it directly and return result, if validation passed;
// return (processed) errors, if validation failed
def bind[T, R2, M <: InputMode](mapping: Mapping[T, M], data: Map[String, String])(consume: T => R2)

to

// bind mappings to data, and return an either, holding validation errors (left) or converted value (right);
// let user to decide how to proceed
def bind[T, M <: InputMode](mapping: Mapping[T, M], data: Map[String, String]): Either[R, T]