Use present tense in messages

I find it a bit more natural if the command tells you what it's doing
and not what it /should/ do.
Check that extension tool exists

Check that gnome-shell-extension-tool actually exists before using it or
installing any extension.
Create extension directory if it doesn't exist

The extension directory might not be available when copying the files
there so create it first.
A readability improving newline

Just a little newline to improve readability of the two install steps.
Merge pull request #1 from moonlite/fix_install_script

Some fixes to install.sh