__getitem__ for 0d arrays returns 0d arrays (not 1d arrays)

This fix ensures that indexing a 0-dimensional array returns a 0-dimensional
numpy array (consistent with numpy's behavior), not a 1-dimensional array with
one element, which is the current behavior.

I'm sure there is a more elegant solution to this issue, but I think this is
one of the better options short of rewriting the core indexing routines. As an
alternative, I did try removing the hack `if nDims == 0: ...` at the start of
_StartCountStride but that definitely broke things. Presumably that hack is
there for a reason?

Note that to fully match how numpy handles 0-dimensional arrays, we would
actually raise an error when somebody tries to access or set a 0-dimensional
variable with `v[:]`. The right syntax for indexing a 0-dimensional array is
somewhat obscure: `v[()]` (using an empty tuple) or `v[...]`. But I avoided
this option because it would assuredly break some user code.

My fix here will probably break some poorly constructed user code, too, if it
relied on the broken 0-d arrays are always 1-d behavior. The right way to
handle this would have been to use `np.asscalar`. So there may be a case for
saving this until the next major version.
Edits in response to comments on PR#220
Merge pull request #220 from shoyer/fix-0d-arrays

__getitem__ for 0d variables returns 0d arrays (not 1d arrays)
potential fix for issue 218
potential fix for issue 218
add comment referencing issue 218
test for issue 218
use weakref.proxy instead of weakref.ref
update
add some comments
remove bogus 'pass'
rename README to README.md
fix typo in comment
use nc-config by default
update url in setup.py to point to github instead of googlecode
don't try to print fill info for non-primitive data types
take out outdated restriction on diskless files
Merge pull request #219 from jswhit/master

potential fix for issue 218 (circular references preventing garbage collection)
modify __unicode__ for Variable and Dimension to return more useful error message when Dataset
object has been garbage collected.
update