Remove username handling inconsistency

In https://bnw.im/p/QO2GG3, @0xd34df00d complained that BnW displays
usernames with leading "@" but requires not to use it it the commands.
This commit removes that inconsistency, making at sign optional in all
the commands that require username as argument.
Test for equivalency of '@user' and 'user'
Merge pull request #84 from Minoru/remove-username-inconsistency

Remove username handling inconsistency