Minor comments improvements in Message, EmptyMessage and NetworkParameters.
Renamed 'bytes', 'msg' to 'payload' in all Message classes.

* This is intended to clarify distinction between full message and its payload to match terminology advertised on https://en.bitcoin.it/wiki/Protocol_specification.
* Does not change public API.
* These refactorings were mostly automated and should not introduce bugs. All tests pass.
Migrate usages of Utils.bytesToHexString() to Utils.HEX.encode(), which in turn uses Guava's Base16 encoding.
Migrate usages of spongycastle Hex.encode/decode to Utils.HEX.encode/decode(), which in turn uses Guava's Base16 encoding.
Arbitrary number of key creation (batch)

Functionality added to create keys in batch (any arbitary number),
previously keys  were created/returned one at a time.
License headers are not meant to be put into JavaDocs.