Backup recovery

This is PR is meant to help users recover their funds is something go wrong with txProposals sync
or any other problem that corrupt the backup.


Fixes https://github.com/bitpay/copay/issues/1148
and https://github.com/bitpay/copay/issues/1147

How to use it?
https://gist.github.com/matiu/3305c42b341916fb7afc#new-method

Also implements `/backup?showPrivate=1`
without interface, so the user can easily recover his private key