Feature/sort index

#### Summary:

Adds functions sort_indices_asc and sort_indices_desc functions.

#### Intended Effect:

Fix issue #554 (feature request).

#### How to Verify:

Unit, unit-rev and unit-fwd test were added and should pass.

#### Side Effects:

No.

#### Documentation:

Documentation added to the manual, would be cool if someone explained the function better than I did there though.

#### Reviewer Suggestions:

Bob Carpenter