Add DUBP Mnemonic SigningKey feature

Add a from_dubp_mnemonic function in SigningKey class to create a DUBP Mnemonic wallet from a 12 words mnemonic passphrase.

Follow the RFC 0014 of the Duniter Project.

changed milestone to %1.0.0

added Authentication Crypto Feature labels

mentioned in commit e74fb6c2

mentioned in commit 02244b29

changed due date to April 05, 2021

mentioned in commit 36e1dcae

mentioned in merge request !125 (closed)

changed due date to April 04, 2021

removed due date

mentioned in commit dd5d0634

mentioned in merge request !130 (merged)

mentioned in commit 2c408d3d

mentioned in commit b0f4a8da

mentioned in commit 79387c95

changed due date to May 06, 2021

mentioned in commit 71b6378b

marked this issue as related to silkaj#90

mentioned in issue silkaj#90

mentioned in commit 6e016734

marked this issue as related to #161 (closed)

mentioned in commit b19b389d

closed with merge request !130 (merged)