Remember me

 - Add a "Remember me" checkbox the login page.
 - ```session.rememberUser()``` sets ```rememberUser``` to true in ```localStorage```.
 - ```session.isPersistent()``` returns true if ```localStorage.rememberUser``` or ```Options.PERSISTENT_SESSION``` are true.
 - ```session.logout()``` sets ```rememberUser``` to false in ```localStorage```.
 - Move ```Options.PERSISTENT_SESSION``` out of ```Wallet``` and only call the functions that used it if ```session.isPersistent()``` is true. Most usages were checking ```Options.PERSISTENT_SESSION``` before calling the functions, then the functions were checking it again. ```session``` can't be used inside ```Wallet```, because ```session``` injects ```Wallet``` and ```Wallet``` should not need to be aware of session state.

Fixes #573.