Add .dropboxignore directory to exclude folders without using selective sync

@Eric W

Asking a company (especially a large one) to "just add" a feature, no matter how apparently small, is going to take a long time. Dropbox is so huge, controlling the security of files for so many other large companies, that they need to implement, test and re-test these features to make sure that they won't mess up their infrastructure.

Would a .dropboxignore particularly mess up their entire infrastructure? Probably not. But any feature still has to be designed, user-tested, developed, tested some more, beta released and then production released. Maybe we will get it by next year if they start now!