Find library code in sites/all/libraries

There have been requests to keep the jsMath library code outside the jsmath module directory, for example to prevent erasure when updating the module. An earlier ticket proposed putting it in the files area, #281367: Put jsMath files in file area. A patch on that ticket had a nifty way to upload the library through the browser.

The developing Libraries API seeks to do something like that, but for libraries in all modules. The attached patch makes jsmath find its files at that path. It also updates the INSTALL.txt to suggest to users to install there. It retains backwards compatibility though and will still find the files if they're stored at their earlier location.