URL safe device identity uses dash delimiter

[deadlyfingers]

There seems to be a '-' in the (URL safe) Device Identity which can cause issues as I use this as a PouchDB Id delimiter.

Possible fixes:

• Instead of using lastIndexOf('-') use second index of '-'.
• Change the encode/decode methods and ensure the '-' is escaped in Device Identity part.

NB: using '.' as url safe character might cause issues if it produces two consecutive dots in the URL.

[deadlyfingers]

Updated encode method:

var URLSafeBase64 = {
  encode: function(string) {
    return encodeURIComponent(btoa(string).replace(/=+$/, ''));
  },
  decode: function(base64) {
    return atob(decodeURIComponent(base64));
  }
};