Modifier arguments should allow ExpressionStatements

This fails to parse:

```
pragma solidity ^0.4.4;

contract A {
    function yes() public constant returns(bool) {
        return true;
    }
}

contract MyContract {
  A a;
  modifier only(bool b) { _; }
  function foo() only(a.yes()) {
  }
}
```

If `a.yes()` is replaced with `true` it parses fine.