Add float_close method.

Wraps floats `a` and `b` in `ndarray`s and calls `Assert.all_close`.
This is helpful for testing standard deviation and other reducer
functions.
Add tests for std. 1d, 2d, 2d with axis, large.
Add standard deviation.
Refactor std tests with randn; add uneven axes.

Splits test_std into `test_std_no_axis` and `test_std_with_axis`. Also
refactor tests to use np.random.randn instead of np.arange.
Refactor _std_mapper and add _num_tiles helper.

_std_mapper still broken for uneven arrays but works correctly for
square arrays.

Add _num_tiles helper function that returns the number of tiles for
necessary for a DistArray, given its shape and the shape of its tiles.
Fix stop overflow in std. for non-square matrices.

Prevents the starting row/col to calculate the standard deviation for
from being greater than the array shape.
Merge pull request #163 from rgardner/add-std-to-builtins

Add std to builtins