Revise/add rake tasks for search index mgmt.  Bump Tire to 0.6.2.

This commit supports the following new workflow for deploying updates containing index changes.

* Offline deployment:
1. install code
2. `rake search:rebuild_all`

* Online deployment:
1. install code
2. `rake search:rebuild_all[2500,3]`
3. `rake search:prune[2500,3]`

Note, 2500 and 3 are batch parameters (batch size, sleep time between batches) that worked well in the loadtest environment.  For edx.org production we may choose to fine tune.

The Tire upgrade ensures we have the most up-to-date code and bug fixes for interacting with ES.

@gwprice @e0d