Operate on unique data for each instance

[danielgtaylor]

This fixes the following bug:

>>> s3 = boto3.resource('s3')
>>> obj1 = s3.Object('my-bucket', 'my-key')
>>> obj2 = s3.Object('my-bucket', 'my-key')
>>> obj1.load()
>>> print(obj2.meta['data'])
{ ... }

The data from obj1 was shared with obj2 which prevented making
multiple unique instances of resources. This fixes the issue by
making each resource operate on a copy of the data, created at
instantiation time. Adds a test to ensure this works as expected.

cc @jamesls, @kyleknap

[coveralls]

Coverage increased (+0.02%) when pulling 060654c on unique-meta into e07c203 on develop.

[jamesls]

[kyleknap]

LGTM 🚢