without underscore.js

To illustrate that this is not necessary (and also not possible with
non-bundled plugins).

to indicate that it's private and to differentiate it from the outer "global"
converse object.

- Remove the `plugins.remove` API method.
- Throw an error when attempting to register multiple plugins with the same name.

updates #752

To illustrate that this is not necessary (and also not possible with
non-bundled plugins).

to indicate that it's private and to differentiate it from the outer "global"
converse object.

Specifically involving querying by the 'xmlns' attribute, where I've had issues
in the past.

after allowing chatting with pending/requesting contacts

Also add `/subject` as alias to `/topic`