Implemented value tangle (#288)

* Refactor: cleaned up unused files

* Refactor: changing branches (need to commit)

* Refactor: refactored some code

* Feat: finished signature scheme for value transactions

* Fix: fix broken merge

* Refactor: refactored some code

* Refactor: refactored transaction marshaling and unmarshaling

* Fix: fixed missing err check

* Feat: added payload string method

* Feat: added Bytes() method to transactionid

* Feat: refactored the marshaling

* Refactor: refactored serialization code

* Feat: started implementing the payload metadata

* Docs: added some doc comments

* Docs: added some additional docs

* Feat: added a CachedObject of the payloadmetadata + added comments

* Feat: updated hive.go + added further models

* Feat: pc died - rescued files from disk

* Feat: further models implemented

* Feat: added missing model for value tangle

* Feat: started writing test cases for value tangle

* Feat: started to adjust marshaling of transaction

* Feat: refactored marshaling of payload

* Feat: intermediary commit before bigger refactor

* Feat: removed identity package

* Fix: fixed bugs due to refactor

* Fix: fixed further bugs

* Fix: fixed further bugs

* Fix: hopefully last bugs :p

* Feat: changed time marshal to use nanoseconds

* Fix: fixed time marshaling

* Fix: fixed serialization bug

* Docs: added a comment to handling the zero value

* Refactor: refactored transaction to separate issuerKey

* Feat: added a parse method for Signatures

* Feat: updated to latest hive.go