Fix/audit suggestions

[Th0rgal]

This pull request implements the changes that were suggested during the audits and code reviews.

Big changes:

limit_price was replaced by an allowance

What it means:

Instead of having a renewing_allowance storage mapping defines as:
(renewer, domain, limit_price) -> 1 or 0
We have:
(renewer, domain) -> limit_price

Why?

• It is actually cheaper. Even though limit_price is a uint256 made of two felts, the high part will always be zero and thus have no associated storage cost. In addition it reduces the amount of hashes required to compute the storage address.
• It means less data to index and makes the logic easier to understand.
• It makes sure one user can only allow one flow of payment.

Replaces limit_price(s) by domain_price(s) in _renew, renew and batch_renew

What it means:

The whitelisted renewer no longer has to specify the yearly allowance set by every user. Instead he has to specify the expected domain_price. The contract will still ensure (domain_price + tax_price) <= allowance so this doesn't change the security guarantees.

Why?

It allows us however to debit less money from users if they allowed us more than needed. As before there is no guarantee about that, but we think having that possibility could be useful to avoid getting too much dust on the contract, especially if the stark domain prices decrease and users didn't have time to update their allowance.

Small changes:

• removed unused imports
• fixed inconsistent sanity checks of span lengths
• fixed incorrect comments & readme
• added a two step process for updating the admin
• added more events for helping user to track the contract state

closes #17

[irisdv]

lgtm!

[irisdv]

it's not the right comment