"requireUsernameProofs" feature flag

Added `requireUsernameProofs` feature flag. When set to `false` username proof is not checked during wallet creation. It's to allow normal (non SDF) stellar-wallet deployments and for stellar-wallet-js-sdk tests to pass (stellar/stellar-wallet-js-sdk#4).
The problem is that stellar-wallet-js-sdk needs this to be set to `false` to run tests. I can set it to `true` but will need to overwrite this file after repository is pulled for tests on travis.