BF: fix broken coverage runs in virtualenv

Now we're running in our own virtualenv, coverage was trying to cover
everything we imported.  I have no idea why that is, but futzing on a VM
similar to the travis machines seemed to recover the right behavior.
BF: set parrec data type to unsigned integers

Consensus seems to be that REC data is in uint format - see
https://github.com/nipy/nibabel/issues/275
NF: add guard for unexpected bit widths

From comment by Eric L
Merge pull request #279 from matthew-brett/parrec-as-uint

MRG: PAR / REC data are unsigned 8 or 16 bit integers

Consensus seems to be that REC data is in uint format - see 
#275

Raise error for data width other than 8 or 16 bit because we haven't any examples to know what these are.
TST: add MINC2 tests with data files

Use submodule to import data for MINC2 tests.
TST: set up installation of h5py

To run MINC2 tests.
NF: add support for V4 of PARREC format

V4 appears to differ only in having fewer general and image information
fields.
NF: add PARREC V4.1 support with tests

Add PARREC 4.1 support from information by Gregory Lee at
https://github.com/nipy/nibabel/pull/277
Merge pull request #277 from matthew-brett/parrec-v4-support

MRG: add support for V4, 4.1 of PARREC format

Versions 4 and 4.1 seem to differ only in in lacking a few 
fields of the 4.2 PAR header.
Merge pull request #280 from matthew-brett/add-minc2-tests

MRG: add MINC2 images repo, run tests on images

Install h5py for full tests using MINC2.
RF+BF: '' from `get_nibabel_data()` when missing

Return empty string from ``get_nibabel_data()`` if directory does not
exist, instead of None.

Doing this means we can use os.path.join on the output even if it the
directory doesn't exist. The resulting filenames don't make sense, but
the decorator checks for this in any case, so the filenames don't get
used.
Merge pull request #282 from matthew-brett/refactor-nibabel-data

MRG: '' from `get_nibabel_data()` when missing

Return empty string from get_nibabel_data() if directory does not exist,
instead of None.
RF+TST: test PARREC array proxy API; add slicing

Add `__getitem__` placeholder method for PARRECArrayProxy, to fulfill
required API, and test with API validators.
NF: add strided_scalar function

Function to return scalar broadcast out to form full array of given
shape.
NF: implement volume by volume reading for PARREC

Use fileslice when we can to do volume-by-volume reading.
TST: PARREC image with varying slope / intercept

Test with ArrayProxy API.
Merge pull request #269 from matthew-brett/parrec-arrayproxy-api

MRG: test and extend PARREC array proxy

I forgot to add the array proxy test for PARRECArrayProxy.

To make the tests pass, add an interface to return slices from the proxied
array.

Optimize this a little using fileslice and some fancy broadcasting tricks.
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'.