Add M:1 support for Bucket:Index mapping

Allow many buckets to be indexed under one index.  This removes
the index flag and replaces it with a `yz_index` property which
is used to store the name of the index for which the bucket
should be indexed under.  If this property is a) non-existant or
b) equal to the default index then the bucket will be consisdered
a non-indexed bucket and the objects' values will not be indexed.

The HTTP API to create an index no longer automatically installs
a bucket property.  For now 2 calls will be required.

Still need to address index deletion.  Right now an index could
be deleted when buckets are still using it.

* Update riak tests to use new index API.  Added `wait_for_index`
  function that can be used instead of sleeps.

* `yz_flag_transitions`

  * Add 1s sleep after `load_data` to give soft-commit change to commit.

  * Assert the return value from `wait_for_aae`.

* `yz_index_amin`

  * Use smaller ring to make test faster.

  * Add 2s sleeps to prevent ring/metadata races.  Revisit this soon.

  * Put assert in `check_list` so on failure you can see actually values.

* `yz_languages`

  * Use smaller ring.

  * Use 1 node cluster since this test isn't really about clustering.

  * Use `wait_for_index` instead of sleeps.

* Ignore unused vars in `yz_pb`.

* Use `wait_for_index` instead of sleeps in `yz_siblings`.
Use correct var in assert, avoid shadow var
Return 409 when deleting index with associated buckets

Don't allow an index to be removed, at the HTTP layer, when it has
associated buckets.

* Add `confirm_delete_409` test case.

* Make sure index name is string when pulling from bucket properties.
  Eventually index name will just be binary everywhere.

* Add check to the DELETE handler.
Replace `wait_for_aae` with `wait_until`

Use the built-in riak-tset `wait_until` functionality instead of using
my own.

Replace sleep with `wait_for_index`.
Update essential test to use `wait_until` and remove sleep
Improve `yz_errors` test

* Remove sleep.

* Randomize calls.

* Reduce ring size, increase handoff concurrency, and reduce
  inactivity timeout to speedup test.
Update the README for one-to-many index change
Merge pull request #90 from basho/rz-one-to-many

1:M index:bucket mapping