Add the ability to create new indexes in a group.

Bump webmock to a newer version so we can call assert_requested
on a stub.
Fix a deprecation warning in mocha.
Test custom mappings for particular indices.
Return an Index from IndexGroup#create_index.

Also pass around base URIs, rather than a reference to the
SearchServer object, to avoid unnecessary coupling.
Pass the field names into a new index
Refactor tests
Repurpose old elasticsearch code for Index

* change the way it's initialised, add a method to make the transition
  easier
* add readers for index_name and field_names to make our existing 
  tests work
* ignore the Rakefile as the code we would change is going away anyway
WIP Swap out Backends for SearchServer

Functional and unit tests passing.
Lots of gnarly code to delete (yay).
Wire new index switching into an integration test.
Tests for new index-switching functionality.
Add tests for cleaning up old indices.
Migrate advanced search test to new indexing.

As part of this, pull out elasticsearch integration test code into a
shared module.
Move remaining integration tests to new indexes.

Also:
  * Remove hard-coded list of sitemap indices
  * Move reindexing into the Index class as `populate_from`
Add a method to list indices in a group.
Add a method to Index to get its real name.
Add integration tests for index groups.
Fix broken advanced search tests.
Remove the elasticsearch admin wrapper.

Hooray!
Remove unused stubbing helpers.
Remove old reindexing code.