Add support for serializing decimals

[jamesls]

Lambda supports serializing decimals to json,
so we need to support that as well in chalice local.

[arundhaj]

Can we support datetime serialization as well?

from datetime import datetime

Append the following

if isinstance(obj, decimal.Decimal):
    return float(obj)
elif isinstance(obj, datetime):
    f = '%Y-%m-%d %H:%M:%S'
    return o.strftime(f)

[jamesls]

This PR is just for chalice local, so I'd prefer to keep parity with what lambda actually does (which doesn't serialize datetimes). Otherwise you'd have behavior where it works in chalice local but fails when you deploy.