Automatic label-based alignment for math and Dataset constructor

[shoyer]

Fixes #186.

This will be a major breaking change for v0.4. For example, we can now do things like this:

In [5]: x = xray.DataArray(range(5), dims='x')

In [6]: x
Out[6]:
<xray.DataArray (x: 5)>
array([0, 1, 2, 3, 4])
Coordinates:
  * x        (x) int64 0 1 2 3 4

In [7]: x[:4] + x[1:]
Out[7]:
<xray.DataArray (x: 3)>
array([2, 4, 6])
Coordinates:
  * x        (x) int64 1 2 3