Make exist calls public

[wwchen]

I want to make some exist calls, but it's private in GetRequest.java

[manolama]

You probably want https://github.com/OpenTSDB/asyncbigtable/blob/master/src/main/java/org/hbase/async/HBaseClient.java#L672 then :)

[wwchen]

I want to be able to call Bigtable's exist, which I believe is called in HBaseRPC's static exist method but it's private.

Context on why I need to call exist functions: issue 1463

[manolama]

Ah thanks for the context! Would you mind opening a PR for it and I'd be happy to merge? Thanks!