Initial Windows installer

Instructions at the top of the installer.nsi file.

Some ideas for additions in the future:

* Adding a configuration file that contains the installation directory of phobos/druntime (needs more robust configuration file support in DCD, particularly loading a config file in the dcd-server.exe's path).
* Adding DCD as a Windows service that starts up when Windows starts up. Probably a lot of multi-user environment issues to consider.
* Adding path to dub installation to a configuration file.  `dub describe` dumps out json with the path to all dependencies of a project so DCD would know where to look for every package dub supplies. dub defaults to installing packages to %APPDATA%\dub\packages on Windows so maybe just recursively loading that will suffice.