Add Windows installer

Just to make it trivial to add dub to the PATH. There is also an optional command line shortcut which adds dub to the PATH of just that command window instance (for people who prefer not to mess with their system PATH).

I can add some graphics which really helps spruce up an installer in my experience (I did that with DMD's and my own commercial products) but I figured I'd see what you thought of this before I whipped some up (I'd just grab your dub logo and make it fit the image size requirements).

There are two options at the top of the installer script.  One to specify the dub version and one to specify the path to the dub executable (which defaults to where build.cmd puts it in the root).

If you haven't used [NSIS](http://nsis.sourceforge.net/Main_Page) before just install it (I recommend using the Large Strings [special build](http://nsis.sourceforge.net/Special_Builds) because the regular NSIS will fail to update the PATH if the PATH is already 1024 characters long or more).  Once it's installed and you've set the version in the `installer.nsi` script to what you want you can just right click on the `installer.nsi` file and select `Compile NSIS Script` (you can select the choose compressor option instead if you'd rather use LZMA instead of zlib).  It can be run from the command line too if you want to automate this.