Better index exists.

Basically, change `yz_index:exists` to check 1) official index list, 2) on-disk and 3) ping if Solr is running. See the commit messages for more detail.

This addresses #249.

There was also a supplementary fix made to the verify script which I broke in a previous commit.There is no longer a top-level `yokozuna` dir in top level working dir. Instead, `riak_yz/deps/yokozuna` is used. You'll need to change your `yz_dir` in the `yz_verify` section of `~/.riak_test.config`.