Improved usage printing

Groups commands and makes the default usage output look less messy:

```
Haxe Library Manager 3.1.0-rc.3 - (c)2006-2013 Haxe Foundation
  Usage: haxelib [command] [options]
  Basic
    install   : install a given library, or all libraries from a hxml file
    upgrade   : upgrade all installed libraries
    update    : update a single library
    remove    : remove a given library/version
    list      : list all installed libraries
    set       : set the current version for a library
  Information
    search    : list libraries matching a word
    info      : list information on a given library
    user      : list information on a given user
    config    : print the repository path
    path      : give paths to libraries
  Development
    submit    : submit or update a library package
    register  : register a new user
    local     : install the specified package locally
    dev       : set the development directory for a given library
  Miscellaneous
    setup     : set the haxelib repository path
    selfupdate: update haxelib itself
    convertxml: convert haxelib.xml file to haxelib.json
    run       : run the specified library with parameters
    git       : use git repository as library
    proxy     : setup the Http proxy
```