Support 'default' rule for sequence and mapping.

Currently 'default' can only be used with scalar types. It should be possible to specify a default value for mappings and sequences too.

See this schema for example:
```yaml
---
type: map
mapping:
eggs:
type: map
mapping:
foo:
type: str
required: true
bar:
type: int
required: true
default:
foo: hello
bar: 42

```
With this schema, empty root mapping should be valid and it would default too:
```yaml
eggs:
foo: hello
bar: 42
```
But if the eggs key is present then both foo and bar must be specified, so this one shouldn't be valid with above schema:
```yaml
eggs:
foo: hello
```