ENH: Provide seperate function for parsing/grouping DICOM files.

Use tuple of keywords instead of format string for grouping. Keywords
that don't exist are set to None (instead of raising an exception).

The CLI now dynamically sets a valid format string if none is specified.
ENH: Use nibabel.orientations to reimplement 'reorder_voxels' function.
ENH: Store reorientation transformation in DcmMeta extension.

This will allow people to update directional meta data from the DICOM to match the Nifti image.
ENH: Make default reorient_transform None, be more carful about merging.

Improved some doc strings and tests too.
DOC: Lots up documentation updates and improvements for 0.6 release.
For now undo conversion of TM values pending further discussion.
ENH: Try to get github to recognize the readme is ReST
DOC: More documentation fixes.

Put docstrings on class level instead of under __init__ method.
BF: Fixed issues from commit d227402
DOC: Trying to allow builds of documentation on ReadTheDocs
DOC: Still trying to get RTD to work...
DOC: Still working on RTD integration...
DOC: ...
DOC: Updated README to point to RTD documentation.
DOC: Add numpydoc in setup.py extras_requires for documentation.

Moved the pip_requirements file for RTD to the doc/ directory.
DOC: Added docstring for NiftiWrapper.from_dicom_wrapper
DOC: Fix so documentation will build locally and on RTD.

Updated README file with more options for documentation, testing,
and installing.
DOC: Document direct access to DcmMetaExtension.

Other doc fixes
Set version for 0.6 release
DOC: Trying to fix API documentation on RTD