[dev] [en] recommendations + text commands

[Goffi]

G'day,

I have added a new "recommendations" field for plugins in PLUGIN_INFO: if a 
plugin is in recommendations instead of dependencies, it will be treated as a 
dependecy (loaded if needed), but if it can't be found no exception will be 
raised (i.e. it's not a mandatory plugin).

A use case is text commands: before, text-commands plugin was implementing 
everything, including commands like "join" which is linked to XEP-0045, so 
XEP-0045 was a dependency of text-commands. Now, text-commands has no more 
dependency, and it's a recommendation of plugin XEP-0045: if it's available, 
XEP-0045 will add its own commands, else there just a warning in the logs 
telling that commands are not available.

2 new methods are in text-commands:

	- registerTextCommands which look in an instance for every method name 
"cmd_.*", and add them as text command (the docstring is used for /help).

	- addWhoIsCb, which can be used by plugin to add information to /whois 
command. Software version add client name/version and OS used with this 
method.

cheers
Goffi