Add support for nested objects

[Joanata]

This PR adds the support for nested objects.

Issue #28

[bsonntag]

With get there's no need to check if errors exists. You can also pass the fallback value as the third argument.

Suggested change

	error: errors ? get(errors, field) : null,
	error: get(errors, field, null),

Please do the same in the two lines below.

[bsonntag]

set mutates the object you pass as the first argument, so this mutates the previous state, which should be avoided. Instead, please pass an empty object as the first argument:

Suggested change

	...set(state, payload.field, payload.value)
	...set({}, payload.field, payload.value)

Please do the same on other set calls you have.

[bsonntag]

I'm not sure about this... What if you have a field in the form named active?

What do you think about keeping the meta object as a flat object with keys with dots? Like this:

meta: {
  'foo.bar': {
    active: false,
    // etc
  }
}

Let's discuss this on Monday, ok?