Add support for percolate during Bulk Index operation

Previously in NEST you could request to percolate during a bulk index operation, but the items collection did not contain a property for the list of matches that ES returns in it's response.  I needed that for a scenario of mine and thought it may be useful to others as well.

FYI I'm a new to git and nunit so if there is a different recommended approach or series of tests you would like I can try to incorporate those.

Thanks!