Adding support for get-proof-by-hash API

[ankishagarwal]

It would be useful to just lookup for inclusion proof by MerkleTreeLeaf Hash.
So I wanted to add the support for get-proof-by-hash API which is currently not available in CT Java client.

This support is currently available in python and Go CT library, want to add that same for Java client.

I have tested this feature locally, works well at the moment. Will raise a PR from my forked repository sooner.

Adding 2 public methods which will accept MerkleTreeLeaf (POJO), MerkleTreeLeaf SHA256 hash respectively.

[ankishagarwal]

Changes merged into master