Re-alphabetize --help
Minor changes to a few messages
Add support for installing from an .iso file

Thanks to Emilio Torres-Manzanera.
Trap some signals and do a clean exit
Clean up on exits
Options --repository and --iso are incompatible

Give error and exit if both are used.
Install tlmgr.desktop system wide

Thanks to Emilio Torres-Manzanera.
Add --help and README info describing --iso option

Thanks to Emilio for catching this omission.
Add support for systems with multiple users
Decrease mirror timeout to 15 seconds

There is a backup that will try 60 seconds if all fail
so we can be a little more aggressive with the first try.
Make script runnable as a stand-alone (fixes #3)

The file debian-control-texlive-in will now be downloaded
if it is not in the directory from which install-tl-ubuntu is run.
It is the only dependency and thus now only install-tl-ubuntu is
needed for things to work. I've learned that many people use Github
by downloading files individually. This commit makes it so they only
need to download one file.

An alternative would be to embed the control file in the main script,
but this would remove the flexibility of modifying the control file
while updating to the latest install-tl-ubuntu. One such use case is
that one might want to modify the Debian control file so that Ubuntu
installs a certain (older) version of a package. This should then
probably (unless you want both versions) be accompanied by a change
in the TeX Live profile (use --profile and see example.profile).
Bump version to 0.4