Fix concatenating of Variables with dtype=datetime64

This is a more direct fix for the bug @akleeman reported #125.

Previously, it did not always work to modify Variable.values in-place (as done
in Variable.concat), because as_array_or_item could return a copy of an
ndarray instead of a view.

@akleeman I didn't incorporate your tests for the
Clean up variable._as_compatible_data
Don't copy datetime64 arrays unless they don't have ns precision
Circumvent pandas dtype inference when indexing coordinates
Fix array_equiv for 0-dimensional object arrays

This is a work-around for this upstream issue:
https://github.com/pydata/pandas/pull/7176
Verify that index/concat does not change dtype for datetime variables
Merge pull request #134 from shoyer/time-conversion-fix-alt

Fix concatenating Variables with dtype=datetime64