{DataArray,Dataset}.indexes no longer creates a new dict

According to this toy benchmark, this shaves off between 20%
(diff-indexes) to 40% (same-indexes) of xray's overhead for
array math:

    import numpy as np
    import xray
    x = np.random.randn(1000, 1000)
    y = np.random.randn(1000, 1000)
    dx = xray.DataArray(x)
    dy = xray.DataArray(y)
    %timeit x + x # raw-numpy
    %timeit dx + dx # same-indexes
    %timeit dx + dy # diff-indexes
Merge pull request #172 from shoyer/Indexes-no-copy

{DataArray,Dataset}.indexes no longer creates a new dict