Improve the script igv.sh

* Add a shebang `#!/bin/sh`.
* Support symlinking the script with `readlink $0`.
* Properly quote the arguments to allow special characters
  by replacing `$*` with `"$@"`.
* Replace the shell process by using `exec`.
* Put the process in the background with `&`.
Merge pull request #54 from sjackman/master

Improve the script igv.sh