Implement crypto signing and verification as researched - close #123, close #128

[kyroy]

Description

Implement crypto signing and verification as researched

Related Issue

123 Find out how to do signing ([with Mist] + ether accounts)

128 Find out how to validate signatures in BC

Motivation and Context

Ease the usage of crypto functions

Notes

Merge #132 first!

Checklist reviewer:

• [x] The code has been peer-reviewed by @graup

[graup]

Would it be possible to re-commit this without all the changes to the contract that somehow ended up in here? That messes up the git history. Basically rebase in a way that doesn't make my Refactor commit part of this branch.

[kyroy]

Done

[graup]

Perfect, thanks!