Handling of apostrophes is incorrect

[nkovacs]

Copied from messageformat/messageformat#175

messageformat.js does not handle double apostrophes correctly.

var mf = new MessageFormat('en');

console.log(mf.compile("I see '{many}'")());
console.log(mf.compile("I said '{''Wow!''}'")());
console.log(mf.compile("I don't know")());
console.log(mf.compile("I don''t know")());

This should output (assuming DOUBLE_OPTIONAL):

1. I see {many}
2. I said {'Wow!'}
3. I don't know
4. I don't know

The first two don't work at all, and the fourth one outputs two single quotes instead of one.

See http://icu-project.org/apiref/icu4j/com/ibm/icu/text/MessagePattern.ApostropheMode.html
and http://userguide.icu-project.org/formatparse/messages#TOC-Quoting-Escaping