Meta settings class

This implements a new syntax for settings on Models, with added support for `host`, `region` settings per Model - as discussed in #9.

**This is not a backward compatible change**

Example:
```python
from pynamodb.models import Model
from pynamodb.attributes import UnicodeAttribute


class UserModel(Model):
    """A user model class"""
    class Meta:
        """A class for settings"""
        table_name = "users"
        # Optional, defaults to us-east-1
        region = "us-west-1"
        # Optional, defaults to the the default provided by botocore
        host = "http://localhost"
    user_name = UnicodeAttribute(hash_key=True)
```