Support for new ERC721 interface

- Tests for new features are pending
- ERC721 is abstract, since it requires metadata implementation
- Move some methods into DeprecatedERC721 contract
- Reorganise base vs full implementation
- Pending tokenByIndex

contracts/AddressUtils.sol

@@ -0,0 +1,11 @@
+pragma solidity ^0.4.18;
+
+library AddressUtils {
+
+  function isContract(address addr) internal view returns (bool) {
+    uint size;
+    assembly { size := extcodesize(addr) }
+    return size > 0;
+  }
+
+}