Update Windows installer defaults

Main changes:
- Does the windows installation with Windowish defaults, see #176 and below (`/P` is no longer a required parameter)
- example added to help text to explain a self contained installation, as the installation is now by default more spread out over the system.

Minor changes
- Adds a prompt before installing, and a switch `/Q` to ignore that
- registry information is now loaded without prompts

This fixes #176

Default locations are now;
- LuaRocks itself (`/P`): `%PROGRAMFILES%\LuaRocks\2.1\` (usually `C:\Program Files\LuaRocks\2.1\` or `C:\Program Files (x86)\LuaRocks\2.1\` on 64bit systems)
- System rock tree (`/TREE`): %PROGRAMDATA%\LuaRocks\` (usually `C:\ProgramData\LuaRocks`)
- System config file  (`/CONFIG`): %PROGRAMDATA%\LuaRocks\` (usually `C:\ProgramData\LuaRocks`)

User rocktree and config file were not changed.