Convert all tabs to spaces

Indentation was a mixture of tabs and spaces.  I converted all tab
characters to spaces.  I know this is a point of contention but code
should consistently use one or the other.
Added a revertChange method

I saw a revertChange function in the commit log but then saw it
removed.  Was it broken?  Here’s another go at it.

revertChange is exposed to the user and revertArrayChange is its helper
function, similar to applyChange+applyArrayChange.  Added a note in the
readme.
Merge pull request #18 from ravishivt/master

Convert all tabs to spaces and added revertChange