Predict safe address util

[DaniSomoza]

What it solves

Resolves #355

How this PR fixes it

Added predictSafeAddress as a util function
Added encodeSetupCallData as a util function

Removed calculateProxyAddress and using now the new predictSafeAddress util function
Removed getSafeInitializer and using now the new encodeSetupCallData util function

Other changes

Now we are using the PREDETERMINED_SALT_NONCE as a default value for the saltNonce in the SafeFactory instead of the old default saltNonce based on the timestamp:

(Date.now() * 1000 + Math.floor(Math.random() * 1000)).toString()

see this comment

Tests

Added predictSafeAddress unit tests: