Allow expressions in modifier arguments

[raineorshine]

Fix for https://github.com/ConsenSys/solidity-parser/issues/27

e.g.

contract MyContract {
  function fun() mymodifier(foo.bar()) {}
}

[tcoulter]

Awesome @raineorshine. Can you add the above code example into https://github.com/ConsenSys/solidity-parser/blob/master/test/doc_examples.sol, which is the catch all for testing to assure we can parse everything in that file?

It's not a great way to test each feature, but it's good enough for now.

[raineorshine]

Ready to merge

[tcoulter]

Thanks @raineorshine!