Add fallback check in ABI validation

[ngotchac]

Fixes #3392

Test with

contract test {
  function() {}
}

For now, the ABI validation ignores the fallback function. Not sure how it can be used.

[coveralls]

Coverage remained the same at 86.012% when pulling 3b8286f on ng-abi-fallback into 039bd3c on master.