allow posting with default index and/or type for bulk requests

Add methods to post bulk requests with a default index/mapping type.
It looks like in the rest of elastisch you've mainly got separate methods for these rather than optional parameters, so I've done the same.
It might be worth pulling out the bulk-delete and bulk-index methods from the test to make it a bit easier on clients building these requests - is there a sensible namespace to put them in?