Provide a help summary in lein help

The help summary for the bikeshed task doesn't show up in `lein help`. This commit fixes that.

Or rather, this:

```
$ lein
Leiningen is a tool for working with Clojure projects.

Several tasks are available:
ancient             Check your Project for outdated Dependencies.
bikeshed
change              Rewrite project.clj by applying a function.
check               Check syntax and warn on reflection.
```

turns into:

```
$ lein help
Leiningen is a tool for working with Clojure projects.

Several tasks are available:
ancient             Check your Project for outdated Dependencies.
bikeshed            Bikesheds your project with totally arbitrary criteria.
change              Rewrite project.clj by applying a function.
check               Check syntax and warn on reflection.
```