optional callback to Accounts.loginWithPassword is no more optional

[splendido]

Hi there!

If I'm not wrong, commit b567046 hampered the possibility to call Accounts.loginWithPassword without callback option.
See lines

meteor/packages/accounts-password/password_client.js

Line 44 in 1c12b1a

callback(error);

and

meteor/packages/accounts-password/password_client.js

Line 46 in 1c12b1a

callback();

I'm not making a pull request since this can be easily fixed in various way, among which, e.g., putting callback && callback(). But it might depends on personal taste...

btw, awesome work!

[estark37]

Thanks @splendido! Fixed on devel.