Update the NSIS installer script.

Update the NSIS installer script. The installer has been changed in the following ways:
 - If the download of a component errors, the user has the option to abort installation, retry the download, or ignore the download.
 - Default to installing Nimrod in the root directory of the current drive.
 - Add various tool paths to the PATH environment variable.
 - Download component archives to a system-designated temporary path, rather than the installation path.
 - The compression method is set to LZMA, instead of the default ZIP