Add ability to reload indexes

Add the ability to reload indexes on a running cluster. This is useful
for loading schema changes on the fly. E.g. adding a field to the
schema.

The index reload call is a blocking call that broadcasts cluster-wide.
The caller should always check the return value. If any errors occurred
then the reload should be retried until all nodes report success.
Otherwise you could end up with different version of the schema across
the nodes.
Use 2-tuple error instead of 3

Be consistent and return a 2-tuple error like everything else in Erlang.
There is no reason to return schema name with error as the caller had to
pass in a schema name to make the call in the first place.
Refactor index admin test

Make things a tad cleaner and fix 404 check.
Rename reload_index to reload

No need to use the _index suffix. This function lives in the yz_index
module.
Merge pull request #260 from basho/feature/rz/index-reload-server

Add ability to reload indexes