Change log.tails and log.heads to both return hashes and add get methods for Entries

[shamb0t]

@satazor has pointed out that currently log.heads return hashes (or cids), log.tails return entries and log.tailHashes (or soon log.tailCids) return hashes/cids while it makes more sense for log.heads and log.tails to return hashes/cids and have log.headEntries and log.tailEntries to return entries. We can track discussion and those changes here.

[aphelionz]

Will we be dropping tails soon since Log.values traverses the log? That might simplify the discussion and decision here.

[aphelionz]

Removing my assignment as I'm not currently working on this.