tweak template descriptions

address #105 

Simplify and clarify the template descriptions a bit.

```
Stagehand will generate the given application type into the current directory.

usage: stagehand <generator-name>
    --[no-]analytics    Opt-out of anonymous usage and crash reporting.
-h, --help              Help!
    --version           Display the version for Stagehand.
    --author            The author name to use for file headers.
                        (defaults to "<your name>")

Available generators:
  consoleapp - A simple command-line application.
  package    - A starting point for Dart libraries or applications.
  polymerapp - A web app built using polymer.dart.
  shelfapp   - A web server written using the shelf package.
  webapp     - A web app for the developer that doesn’t want to be confused by too much going on.
```

@sethladd 
Hmm, just `webserver`? Nothing shelf specific? `shelf_server`? I know we kick a few ideas around in the past.