add putUser() to update user metadata

[jgillich]

No description provided.

[nolanlawson]

Looks like a great PR! I will look it over and merge soon.

[staxmanade]

This is obvious to couchdb pro's but being a noob - it would be helpful to have 2 error messages here...

Possibly

if (reservedWords.indexOf(key) !== -1) {
  return callback(new AuthError('Cannot use reserved word in metadata: "' + key + '"'));
}
if (key.startsWith('_')) {
  return callback(new AuthError('Cannot have metadata key start with an _ as seen on key: "' + key + '"'));
}

Thoughts?

[jgillich]

Hmm, maybe? But that could be done in another PR.