DOC: typo in comment
NF: add in_memory property

`in_memory` property is True if image has an array as its data object,
or if a proxy image has been cached.
NF: add "caching" flag to ``get_data``

Caching flag can take values 'fill' and 'unchanged'.
RF: docstring rewrite; a PEP8 fix

Suggested by Eric L from PR review.
Merge pull request #276 from matthew-brett/in-memory

MRG: API to help modify image caching

Add 'caching='fill'|'unchanged' keyword to get_data method.

Add in_memory property to tell user whether image already has an in-memory copy of the data.