[ADDED] Address validation script

Will allow admins to run a validateaddress check via RPC calls against
the coind for all currently available users in the accounts table.

* UNSET for users without a coin_address
* INVALID for addresses not validated against the RPC
* VALID for addresses that validated fine

Just to give people a small tool at hands. Usually addresses are already
validated during account edit, so this script is mostly for those pools
that have not had that change yet and wish to notify their users.

Addresses #1177
[ADDED] Payout fail hint: Wallet covers balance?
Merge pull request #1275 from TheSerapher/issue-1177

Issue 1177