Implement gaiacli sign for manual signing of transactions

[jackzampolin]

ref #966

This issue is part of implementing the generate, sign, send workflow for offline transaction generation. It should also be a place to discuss implementation details for gaiacli sign

Description

As a gaiacli user
I want to sign a Tx that was generated beforehand
So that I can review the contents of the transaction before broadcasting it.

Acceptance Criteria AC1

Given a transaction file generated with gaiacli --generate-only and an account name
When I run gaiacli sign
Then a signature is generated with the given account's key and
it is attached to the transaction and
its JSON representation is printed out.

Acceptance Criteria AC2

Given a transaction file generated with gaiacli --generate-only and a HD key
When I run gaiacli sign
Then a signature is generated with the given key and
it is attached to the transaction and
its JSON representation is printed out.

[alessio]

Description

As a gaiacli user
I want to sign a Tx that was generated beforehand
So that I can review the contents of the transaction before broadcasting it.

Acceptance Criteria AC1

Given an unsigned transaction and an account name
When I run gaiacli sign
Then a text representation of the signed transaction is printed out.

Acceptance Criteria AC2

Given an unsigned transaction and a HD key
When I run gaiacli sign
Then a text representation of the signed transaction is printed out.