Remove ownable, simplify minter protection

[ongrid]

1. Token should have a single non-changeable minter received as constructor argument on deployment;
2. Inherirance from Ownable should be removed;
3. Remove minting finalization (event MintFinished, bool public mintingFinished, modifier canMint, modifier hasMintPermission, function finishMinting);
4. mint should be decorated by onlyMinter modifier (just like onlyOwner in Ownable);

  /**
   * @dev Throws if called by any account other than the minter.
   */
  modifier onlyMinter() {
    require(msg.sender == minter);
    _;
  }