Resurrect AddIndex() and CreateIndex()

Indexes are critical for performance.

In this PR I've resurrected the "index management" pull request below from https://github.com/dmajere and updated it with some documentation in the README.  Feel free to squash/cherrypick/whatever if I haven't prepared it just right, as I'm not the best with git.

https://github.com/coopernurse/gorp/pull/97  was the original.