cmd: add method RegisterTopic to Manager

This method will register help topics. The content of each topic will be
displayed by the "tsuru help" command. For example, when registering the
topic "target" with the content "remote tsuru server\n\nsee target-set
and target-add for details", tsuru users will be able to see this
content by running:

	% tsuru help target
	remote tsuru server

	see target-set and target-add for details

It will work like in go, where we have "go help testflag" and "go help
gopath".

The motivation comes from #453. I've fix that isse by changing the error
message, but we should have a "tsuru help target" command, that explains
what's a target and which commands manipulate them.
cmd: fix "Error:" duplication in help output when command doesn't exist

Before:

	% tsuru help asihfahisfa
	Error: Error: command "asihfahisfa" does not exist.

Now:

	% tsuru help asihfahisfa
	Error: command "asihfahisfa" does not exist.
cmd: add support for topics in the help command

It will first check for commands, and them for topics. Commands are more
important, and we allow users of the package to register a topic with
the same name of an existing command.
cmd: include topics in the output for the naked call
cmd: improve output for naked help calls
cmd: register topic for target management
cmd: improve error message for undefined targets

The topic "target" is way more helpful.