API: Add equals method to NDFrames. (Implemented with `array_equivalent`, which is similar to `np.array_equal` except that it handles object arrays and treats NaNs in corresponding locations as equal.

TST: Add tests for NDFrame.equals and BlockManager.equals

DOC: Mention the equals method in basics, release and v.0.13.1
Merge pull request #5283 from unutbu/array-equivalent

API: Add equals method to NDFrames.