Fix getindex for Number-specific DataArray

The getindex methods for DataArray's containing Number-type data
did not check for #undef in data, and failed on cases like

x = DataArray(Integer, 4)
x[[2, 3]]

The fix removes the specialized methods such that getindex falls
back to more generic implementations, where data is only copied
if not marked as NA.
Removed surperfluous actions in getindex for DataArray

getindex for DataArray with multiple indicies set all NA elements
explicitly to NA, even though this is already done when creating
the new empty DataArray with similar(). This superfluous actions
has been removed.
Merge pull request #83 from jdrugo/fixgetindexundef

Cleaning up getindex for DataArray