Add Polkajs provider for Substrate

[Rgascoin]

Feat: Providers are not handled by Substrate account
Solution: Add Polka.js support
(related to: #60)

Important Notes:

• At this moment, the Aleph network won't be able to validate messages from provided Dot Account, until nodes upgrade their dependency substrateinterface
• This feature is not tested, because it can only be run in the browser and it doesn't appear to have any documented tool for mocking polkadotjs extension.

Link to the PyAleph PR: aleph-im/pyaleph#358

How I verified the reliance of this feature:

• I modify a local Aleph node with substrateinterface in 1.4.1
• The Sign() method of Substrate have a new verification step to check the integrity of the signature using signatureVerify()
• Make some manuals tests with toolshed

More about the polkadot version:

• https://github.com/polkascan/py-substrate-interface/releases/tag/v1.2.0
• False negative verifying a message signed with polkadot{.js} polkascan/py-substrate-interface#185

[BjrInt]

Waiting for the update on pyaleph before merging this.

[BjrInt]

Merged into pyaleph, waiting for the next pyaleph release.