Added support for named exports

[stofolus]

I've encountered an unwanted warning when you use a named export:

export function calc() {
    doStuff();
}

Default exports were already supported so I figured it should be handled in the same way.

[stofolus]

Now it looks at the right node or returns null. I've also updated the ExportDefaultDeclarationNodeMatcher to do the same.

I hope this is the correct behaviour!

Thanks for you help and patience. I haven't worked with an AST tree before.

[petebacondarwin]

Oops I merged but realised that the commit messages are not in the correct format. I am going to revert and fix up.