Export feature

Hi!

This PR implements a feature to export all bookmarks in a Firefox-like bookmarks.html file. UI isn't really beautiful (I've added a button at the bottom of the page) but I didn't really need more than that. Apparently tags are not parsed when importing bookmarks (seems that Firefox behaves the same, that seems like a bug \o/).

Along this PR, there is a slight refactoring of the server app:
- unless I am wrong, there's no script to compile automatically the server coffee files, so in that case it seems better just to rip off all the JS files and use coffee for the server. I think the cost of parsing the coffeescript is higher at startup, but for an app of that size, it doesn't really matter.
- factored out error handling in controllers/bookmarks and made the control flow easier to read

Feel free to accept / refuse / cherry-pick ;)
Cheers!