Added handling for description in model definitions

[martinlatrille]

Allow to set a description when declaring a definition of a Schema (as described here).

spec.definition(
    'Pet',
    schema=PetSchema,
    description="An animal which lives with humans."
)

"Pet": {
    "description": "An animal which lives with humans.",
    "properties": {...},
    ...
}

[sloria]

Looks good. Thanks!