Make the executable name global

Many commands (especially help commands) use them, so get it once and
use everywhere.
Add update command

Add command that updates an installed tool by removing its executables,
using `git pull' to update the code and then reinstalling the
executables (plus any new ones).
Rename executable_name -> _sti_executable

`executable_name' conflicts with a variable used in one of the commands.
To make it clear that it is a special name add a `_' in front of it.
Show version numbers for certain operations
Fold last line of update help text
Add newlines at end of version messages
Add more docs