Update installation instructions

My thanks go to @stiletto, who wrote original part of it, @kagami, who
wrote another part, and aurora @ bnw.im, who was the lab rat we tested
it all on. Thank you!
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.
Merge pull request #83 from Minoru/update-readme

Update installation instructions
Test for equivalency of '@user' and 'user'
Merge pull request #84 from Minoru/remove-username-inconsistency

Remove username handling inconsistency