Add two places where instructions will be displayed: after download, and in a downloaded README file.

This partially addresses #905.  README content still TBD (but can be done by others, now that the structure is there).

Also changed script names to include OS name, to be easier to identify.

Testing:
* Browse to /install/thankyou/ to see the raw thankyou page.
* Go through the install wizard (both single-server and multi-server editions); you should go to the thankyou page when download is ready, and it should trigger the download
* After downloading, if you unpack the zip, there should be a README file (with minimal content)

![image](https://f.cloud.github.com/assets/4072455/1488258/4308bc22-4745-11e3-8855-5b312736fdfc.png)