Rename percentile to quantile in processor 0.0.2

As the protobuf representation already uses quantile, this change simply
consolidates the naming in the JSON representation.

@matttproud Clients (the ruby client for example) have no way to control the naming of quantile/percentile in the JSON representation, so that change simply aligns the naming with clients supporting protobufs already (will come soon).

Given all currently used clients should be either on version 0.0.1 or support protobufs already, that change shouldn't have any impact.