NF: get_unscaled method for Analyze array proxy

Add get_unscaled for those of us who like to get the raw unscaled data
from the array.
TST: tests for read_img_data function

Test the behavior of the `read_img_data` function as of commit 6d8f6c6.
This is before the changes to Analyze image types to make the header
slope, intercept and data offset be consumed during write, and reset to
default values.  These tests pass against that commit but fail against
current master, which is 62f4c70.
RF: replicate read_img_data behavior + deprecate

Do approximate replication of the old behavior of read_img_data.

Deprecate with suggestions about alternatives.
Merge pull request #224 from matthew-brett/reworking-raw-scaling

MRG: fix and deprecate read_img_data function

The read_img_data function was a hack to help Michael get the raw image
data out of Nifti images without applying the image scaling. The new
API for consumable values in the header broke this code, but it was a
bad idea in the first place and it was in any case broken for MINC.

Add a 'get_unscaled' method for the ArrayProxy object to give a cleaner
API to get the unscaled data. Try and replicate the old behavior of
read_img_data but deprecate so we don't need to maintain it long term.